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2015-07-22AioContext: optimize clearing the EventNotifierPaolo Bonzini
It is pretty rare for aio_notify to actually set the EventNotifier. It can happen with worker threads such as thread-pool.c's, but otherwise it should never be set thanks to the ctx->notify_me optimization. The previous patch, unfortunately, added an unconditional call to event_notifier_test_and_clear; now add a userspace fast path that avoids the call. Note that it is not possible to do the same with event_notifier_set; it would break, as proved (again) by the included formal model. This patch survived over 3000 reboots on aarch64 KVM. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Message-id: 1437487673-23740-7-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-07-22AioContext: fix broken placement of event_notifier_test_and_clearPaolo Bonzini
event_notifier_test_and_clear must be called before processing events. Otherwise, an aio_poll could "eat" the notification before the main I/O thread invokes ppoll(). The main I/O thread then never wakes up. This is an example of what could happen: i/o thread vcpu thread worker thread --------------------------------------------------------------------- lock_iothread notify_me = 1 ... unlock_iothread bh->scheduled = 1 event_notifier_set lock_iothread notify_me = 3 ppoll notify_me = 1 aio_dispatch aio_bh_poll thread_pool_completion_bh bh->scheduled = 1 event_notifier_set node->io_read(node->opaque) event_notifier_test_and_clear ppoll *** hang *** "Tracing" with qemu_clock_get_ns shows pretty much the same behavior as in the previous bug, so there are no new tricks here---just stare more at the code until it is apparent. One could also use a formal model, of course. The included one shows this with three processes: notifier corresponds to a QEMU thread pool worker, temporary_waiter to a VCPU thread that invokes aio_poll(), waiter to the main I/O thread. I would be happy to say that the formal model found the bug for me, but actually I wrote it after the fact. This patch is a bit of a big hammer. The next one optimizes it, with help (this time for real rather than a posteriori :)) from another, similar formal model. Reported-by: Richard W. M. Jones <rjones@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Message-id: 1437487673-23740-6-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-07-22AioContext: fix broken ctx->dispatching optimizationPaolo Bonzini
This patch rewrites the ctx->dispatching optimization, which was the cause of some mysterious hangs that could be reproduced on aarch64 KVM only. The hangs were indirectly caused by aio_poll() and in particular by flash memory updates's call to blk_write(), which invokes aio_poll(). Fun stuff: they had an extremely short race window, so much that adding all kind of tracing to either the kernel or QEMU made it go away (a single printf made it half as reproducible). On the plus side, the failure mode (a hang until the next keypress) made it very easy to examine the state of the process with a debugger. And there was a very nice reproducer from Laszlo, which failed pretty often (more than half of the time) on any version of QEMU with a non-debug kernel; it also failed fast, while still in the firmware. So, it could have been worse. For some unknown reason they happened only with virtio-scsi, but that's not important. It's more interesting that they disappeared with io=native, making thread-pool.c a likely suspect for where the bug arose. thread-pool.c is also one of the few places which use bottom halves across threads, by the way. I hope that no other similar bugs exist, but just in case :) I am going to describe how the successful debugging went... Since the likely culprit was the ctx->dispatching optimization, which mostly affects bottom halves, the first observation was that there are two qemu_bh_schedule() invocations in the thread pool: the one in the aio worker and the one in thread_pool_completion_bh. The latter always causes the optimization to trigger, the former may or may not. In order to restrict the possibilities, I introduced new functions qemu_bh_schedule_slow() and qemu_bh_schedule_fast(): /* qemu_bh_schedule_slow: */ ctx = bh->ctx; bh->idle = 0; if (atomic_xchg(&bh->scheduled, 1) == 0) { event_notifier_set(&ctx->notifier); } /* qemu_bh_schedule_fast: */ ctx = bh->ctx; bh->idle = 0; assert(ctx->dispatching); atomic_xchg(&bh->scheduled, 1); Notice how the atomic_xchg is still in qemu_bh_schedule_slow(). This was already debated a few months ago, so I assumed it to be correct. In retrospect this was a very good idea, as you'll see later. Changing thread_pool_completion_bh() to qemu_bh_schedule_fast() didn't trigger the assertion (as expected). Changing the worker's invocation to qemu_bh_schedule_slow() didn't hide the bug (another assumption which luckily held). This already limited heavily the amount of interaction between the threads, hinting that the problematic events must have triggered around thread_pool_completion_bh(). As mentioned early, invoking a debugger to examine the state of a hung process was pretty easy; the iothread was always waiting on a poll(..., -1) system call. Infinite timeouts are much rarer on x86, and this could be the reason why the bug was never observed there. With the buggy sequence more or less resolved to an interaction between thread_pool_completion_bh() and poll(..., -1), my "tracing" strategy was to just add a few qemu_clock_get_ns(QEMU_CLOCK_REALTIME) calls, hoping that the ordering of aio_ctx_prepare(), aio_ctx_dispatch, poll() and qemu_bh_schedule_fast() would provide some hint. The output was: (gdb) p last_prepare $3 = 103885451 (gdb) p last_dispatch $4 = 103876492 (gdb) p last_poll $5 = 115909333 (gdb) p last_schedule $6 = 115925212 Notice how the last call to qemu_poll_ns() came after aio_ctx_dispatch(). This makes little sense unless there is an aio_poll() call involved, and indeed with a slightly different instrumentation you can see that there is one: (gdb) p last_prepare $3 = 107569679 (gdb) p last_dispatch $4 = 107561600 (gdb) p last_aio_poll $5 = 110671400 (gdb) p last_schedule $6 = 110698917 So the scenario becomes clearer: iothread VCPU thread -------------------------------------------------------------------------- aio_ctx_prepare aio_ctx_check qemu_poll_ns(timeout=-1) aio_poll aio_dispatch thread_pool_completion_bh qemu_bh_schedule() At this point bh->scheduled = 1 and the iothread has not been woken up. The solution must be close, but this alone should not be a problem, because the bottom half is only rescheduled to account for rare situations (see commit 3c80ca1, thread-pool: avoid deadlock in nested aio_poll() calls, 2014-07-15). Introducing a third thread---a thread pool worker thread, which also does qemu_bh_schedule()---does bring out the problematic case. The third thread must be awakened *after* the callback is complete and thread_pool_completion_bh has redone the whole loop, explaining the short race window. And then this is what happens: thread pool worker -------------------------------------------------------------------------- <I/O completes> qemu_bh_schedule() Tada, bh->scheduled is already 1, so qemu_bh_schedule() does nothing and the iothread is never woken up. This is where the bh->scheduled optimization comes into play---it is correct, but removing it would have masked the bug. So, what is the bug? Well, the question asked by the ctx->dispatching optimization ("is any active aio_poll dispatching?") was wrong. The right question to ask instead is "is any active aio_poll *not* dispatching", i.e. in the prepare or poll phases? In that case, the aio_poll is sleeping or might go to sleep anytime soon, and the EventNotifier must be invoked to wake it up. In any other case (including if there is *no* active aio_poll at all!) we can just wait for the next prepare phase to pick up the event (e.g. a bottom half); the prepare phase will avoid the blocking and service the bottom half. Expressing the invariant with a logic formula, the broken one looked like: !(exists(thread): in_dispatching(thread)) => !optimize or equivalently: !(exists(thread): in_aio_poll(thread) && in_dispatching(thread)) => !optimize In the correct one, the negation is in a slightly different place: (exists(thread): in_aio_poll(thread) && !in_dispatching(thread)) => !optimize or equivalently: (exists(thread): in_prepare_or_poll(thread)) => !optimize Even if the difference boils down to moving an exclamation mark :) the implementation is quite different. However, I think the new one is simpler to understand. In the old implementation, the "exists" was implemented with a boolean value. This didn't really support well the case of multiple concurrent event loops, but I thought that this was okay: aio_poll holds the AioContext lock so there cannot be concurrent aio_poll invocations, and I was just considering nested event loops. However, aio_poll _could_ indeed be concurrent with the GSource. This is why I came up with the wrong invariant. In the new implementation, "exists" is computed simply by counting how many threads are in the prepare or poll phases. There are some interesting points to consider, but the gist of the idea remains: 1) AioContext can be used through GSource as well; as mentioned in the patch, bit 0 of the counter is reserved for the GSource. 2) the counter need not be updated for a non-blocking aio_poll, because it won't sleep forever anyway. This is just a matter of checking the "blocking" variable. This requires some changes to the win32 implementation, but is otherwise not too complicated. 3) as mentioned above, the new implementation will not call aio_notify when there is *no* active aio_poll at all. The tests have to be adjusted for this change. The calls to aio_notify in async.c are fine; they only want to kick aio_poll out of a blocking wait, but need not do anything if aio_poll is not running. 4) nested aio_poll: these just work with the new implementation; when a nested event loop is invoked, the outer event loop is never in the prepare or poll phases. The outer event loop thus has already decremented the counter. Reported-by: Richard W. M. Jones <rjones@redhat.com> Reported-by: Laszlo Ersek <lersek@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Message-id: 1437487673-23740-5-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-07-22aio-win32: reorganize polling loopPaolo Bonzini
Preparatory bugfixes and tweaks to the loop before the next patch: - disable dispatch optimization during aio_prepare. This fixes a bug. - do not modify "blocking" until after the first WaitForMultipleObjects call. This is needed in the next patch. - change the loop to do...while. This makes it obvious that the loop is always entered at least once. In the next patch this is important because the first iteration undoes the ctx->notify_me increment that happened before entering the loop. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Message-id: 1437487673-23740-4-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-04-28AioContext: acquire/release AioContext during aio_pollPaolo Bonzini
This is the first step in pushing down acquire/release, and will let rfifolock drop the contention callback feature. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Message-id: 1424449612-18215-3-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2014-12-10block: Use g_new0() for a bit of extra type checkingMarkus Armbruster
g_new(T, 1) is safer than g_malloc(sizeof(T)), because it returns T * rather than void *, which lets the compiler catch more type errors. Missed in commit 02c4f26. Signed-off-by: Markus Armbruster <armbru@redhat.com> Message-id: 1417697709-13087-1-git-send-email-armbru@redhat.com Signed-off-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2014-09-22aio-win32: avoid out-of-bounds access to the events arrayPaolo Bonzini
If ret is WAIT_TIMEOUT and there was an event returned by select(), we can write to a location after the end of the array. But in that case we can retry the WaitForMultipleObjects call with the same set of events, so just move the event[ret - WAIT_OBJECT_0] assignment inside the existin conditional. Reported-by: TeLeMan <geleman@gmail.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: TeLeMan <geleman@gmail.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-09-22aio-win32: fix uninitialized use of have_select_reventsPaolo Bonzini
Always initialize it with the return value of aio_prepare. Reported-by: TeLeMan <geleman@gmail.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-08-29aio-win32: add support for socketsPaolo Bonzini
Uses the same select/WSAEventSelect scheme as main-loop.c. WSAEventSelect() is edge-triggered, so it cannot be used directly, but it is still used as a way to exit from a blocking g_poll(). Before g_poll() is called, we poll sockets with a non-blocking select() to achieve the level-triggered semantics we require: if a socket is ready, the g_poll() is made non-blocking too. Based on a patch from Or Goshen. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-08-29AioContext: introduce aio_preparePaolo Bonzini
This will be used to implement socket polling on Windows. On Windows, select() and g_poll() are completely different; sockets are polled with select() before calling g_poll, and the g_poll must be nonblocking if select() says a socket is ready. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-08-29aio-win32: add aio_set_dispatching optimizationPaolo Bonzini
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-08-29AioContext: export and use aio_dispatchPaolo Bonzini
So far, aio_poll's scheme was dispatch/poll/dispatch, where the first dispatch phase was used only in the GSource case in order to avoid a blocking poll. Earlier patches changed it to dispatch/prepare/poll/dispatch, where prepare is aio_compute_timeout. By making aio_dispatch public, we can remove the first dispatch phase altogether, so that both aio_poll and the GSource use the same prepare/poll/dispatch scheme. This patch breaks the invariant that aio_poll(..., true) will not block the first time it returns false. This used to be fundamental for qemu_aio_flush's implementation as "while (qemu_aio_wait()) {}" but no code in QEMU relies on this invariant anymore. The return value of aio_poll() is now comparable with that of g_main_context_iteration. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-08-29AioContext: run bottom halves after pollingPaolo Bonzini
Make the dispatching phase the same before blocking and afterwards. The next patch will make aio_dispatch public and use it directly for the GSource case, instead of aio_poll. aio_poll can then be simplified heavily. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-08-29aio-win32: Factor out duplicate code into aio_dispatch_handlersPaolo Bonzini
Later, the call to aio_dispatch will move int the GSource wrapper, while the standalone case will still be call the component functions aio_bh_poll, aio_dispatch_handlers and timerlistgroup_run_timers. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-08-29aio-win32: Evaluate timers after handlesPaolo Bonzini
This is similar to what aio_poll does in the stand-alone case. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-08-29AioContext: take bottom halves into account when computing aio_poll timeoutPaolo Bonzini
Right now, QEMU invokes aio_bh_poll before the "poll" phase of aio_poll. It is simpler to do it afterwards and skip the "poll" phase altogether when the OS-dependent parts of AioContext are invoked from GSource. This way, AioContext behaves more similarly when used as a GSource vs. when used as stand-alone. As a start, take bottom halves into account when computing the poll timeout. If a bottom half is ready, do a non-blocking poll. As a side effect, this makes idle bottom halves work with aio_poll; an improvement, but not really an important one since they are deprecated. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-07-09block: drop aio functions that operate on the main AioContextPaolo Bonzini
The main AioContext should be accessed explicitly via qemu_get_aio_context(). Most of the time, using it is not the right thing to do. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2013-12-06aio: make aio_poll(ctx, true) block with no fdsStefan Hajnoczi
This patch drops a special case where aio_poll(ctx, true) returns false instead of blocking if no file descriptors are waiting on I/O. Now it is possible to block in aio_poll() to wait for aio_notify(). This change eliminates busy waiting. bdrv_drain_all() used to rely on busy waiting to completed throttled I/O requests but this is no longer required so we can simplify aio_poll(). Note that aio_poll() still returns false when aio_notify() was used. In other words, stopping a blocking aio_poll() wait is not considered making progress. Adjust test-aio /aio/bh/callback-delete/one which assumed aio_poll(ctx, true) would immediately return false instead of blocking. Reviewed-by: Alex Bligh <alex@alex.org.uk> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2013-08-22aio-win32: replace incorrect AioHandler->opaque usage with ->eStefan Hajnoczi
The AioHandler->opaque field does not exist in aio-win32.c. The code that uses it was incorrectly copied from aio-posix.c. For Windows we can use AioHandler->e to match against AioContext->notifier. This patch fixes the Windows build for aio-win32.o. Reviewed-by: Stefan Weil <sw@weilnetz.de> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2013-08-22aio / timers: Convert aio_poll to use AioContext timers' deadlineAlex Bligh
Convert aio_poll to use deadline based on AioContext's timers. aio_poll has been changed to return accurately whether progress has occurred. Prior to this commit, aio_poll always returned true if g_poll was entered, whether or not any progress was made. This required a change to tests/test-aio.c where an assert was backwards. Signed-off-by: Alex Bligh <alex@alex.org.uk> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2013-08-19aio: drop io_flush argumentStefan Hajnoczi
The .io_flush() handler no longer exists and has no users. Drop the io_flush argument to aio_set_fd_handler() and related functions. The AioFlushEventNotifierHandler and AioFlushHandler typedefs are no longer used and are dropped too. Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2013-08-19aio: stop using .io_flush()Stefan Hajnoczi
Now that aio_poll() users check their termination condition themselves, it is no longer necessary to call .io_flush() handlers. The behavior of aio_poll() changes as follows: 1. .io_flush() is no longer invoked and file descriptors are *always* monitored. Previously returning 0 from .io_flush() would skip this file descriptor. Due to this change it is essential to check that requests are pending before calling qemu_aio_wait(). Failure to do so means we block, for example, waiting for an idle iSCSI socket to become readable when there are no requests. Currently all qemu_aio_wait()/aio_poll() callers check before calling. 2. aio_poll() now returns true if progress was made (BH or fd handlers executed) and false otherwise. Previously it would return true whenever 'busy', which means that .io_flush() returned true. The 'busy' concept no longer exists so just progress is returned. Due to this change we need to update tests/test-aio.c which asserts aio_poll() return values. Note that QEMU doesn't actually rely on these return values so only tests/test-aio.c cares. Note that ctx->notifier, the EventNotifier fd used for aio_notify(), is now handled as a special case. This is a little ugly but maintains aio_poll() semantics, i.e. aio_notify() does not count as 'progress' and aio_poll() avoids blocking when the user has not set any fd handlers yet. Patches after this remove .io_flush() handler code until we can finally drop the io_flush arguments to aio_set_fd_handler() and friends. Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2013-01-17aio: Fix return value of aio_poll()Kevin Wolf
aio_poll() must return true if any work is still pending, even if it didn't make progress, so that bdrv_drain_all() doesn't stop waiting too early. The possibility of stopping early occasionally lead to a failed assertion in bdrv_drain_all(), when some in-flight request was missed and the function didn't really drain all requests. In order to make that change, the return value as specified in the function comment must change for blocking = false; fortunately, the return value of blocking = false callers is only used in test cases, so this change shouldn't cause any trouble. Cc: qemu-stable@nongnu.org Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2012-12-19misc: move include files to include/qemu/Paolo Bonzini
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2012-12-19block: move include files to include/block/Paolo Bonzini
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2012-11-26aio: avoid livelock behavior for Win32Paolo Bonzini
The repeated calls to WaitForMultipleObjects may cause a livelock in aio_poll, where no progress is made on bottom halves. This patch matches the behavior of the POSIX code. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
2012-10-30aio: call aio_notify after setting I/O handlersPaolo Bonzini
In the current code, this is done by qemu_set_fd_handler2, which is called by qemu_aio_set_fd_handler. We need to keep the same behavior even after removing the call to qemu_set_fd_handler2. Reviewed-by: Anthony Liguori <aliguori@us.ibm.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2012-10-30aio: make AioContexts GSourcesPaolo Bonzini
This lets AioContexts be used (optionally) with a glib main loop. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2012-10-30aio: add Win32 implementationPaolo Bonzini
The Win32 implementation will only accept EventNotifiers, thus a few drivers are disabled under Windows. EventNotifiers are a good match for the GSource implementation, too, because the Win32 port of glib allows to place their HANDLEs in a GPollFD. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>