From be2cdc5e352eb28b4ff631f053a261d91e6af78e Mon Sep 17 00:00:00 2001 From: "Emilio G. Cota" Date: Wed, 26 Jul 2017 16:58:05 -0400 Subject: tcg: track TBs with per-region BST's MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This paves the way for enabling scalable parallel generation of TCG code. Instead of tracking TBs with a single binary search tree (BST), use a BST for each TCG region, protecting it with a lock. This is as scalable as it gets, since each TCG thread operates on a separate region. The core of this change is the introduction of struct tcg_region_tree, which contains a pointer to a GTree and an associated lock to serialize accesses to it. We then allocate an array of tcg_region_tree's, adding the appropriate padding to avoid false sharing based on qemu_dcache_linesize. Given a tc_ptr, we first find the corresponding region_tree. This is done by special-casing the first and last regions first, since they might be of size != region.size; otherwise we just divide the offset by region.stride. I was worried about this division (several dozen cycles of latency), but profiling shows that this is not a fast path. Note that region.stride is not required to be a power of two; it is only required to be a multiple of the host's page size. Note that with this design we can also provide consistent snapshots about all region trees at once; for instance, tcg_tb_foreach acquires/releases all region_tree locks before/after iterating over them. For this reason we now drop tb_lock in dump_exec_info(). As an alternative I considered implementing a concurrent BST, but this can be tricky to get right, offers no consistent snapshots of the BST, and performance and scalability-wise I don't think it could ever beat having separate GTrees, given that our workload is insert-mostly (all concurrent BST designs I've seen focus, understandably, on making lookups fast, which comes at the expense of convoluted, non-wait-free insertions/removals). Reviewed-by: Richard Henderson Reviewed-by: Alex Bennée Signed-off-by: Emilio G. Cota Signed-off-by: Richard Henderson --- include/exec/tb-context.h | 1 - 1 file changed, 1 deletion(-) (limited to 'include/exec/tb-context.h') diff --git a/include/exec/tb-context.h b/include/exec/tb-context.h index 1d41202485..d8472c88fb 100644 --- a/include/exec/tb-context.h +++ b/include/exec/tb-context.h @@ -31,7 +31,6 @@ typedef struct TBContext TBContext; struct TBContext { - GTree *tb_tree; struct qht htable; /* any access to the tbs or the page table must use this lock */ QemuMutex tb_lock; -- cgit v1.2.3