On 02/26/2018 09:39 PM, Emilio G. Cota wrote: > 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). > > Signed-off-by: Emilio G. Cota <c...@braap.org> > --- > accel/tcg/cpu-exec.c | 2 +- > accel/tcg/translate-all.c | 101 ++++-------------------- > include/exec/exec-all.h | 1 - > include/exec/tb-context.h | 1 - > tcg/tcg.c | 191 > ++++++++++++++++++++++++++++++++++++++++++++++ > tcg/tcg.h | 6 ++ > 6 files changed, 213 insertions(+), 89 deletions(-)
Reviewed-by: Richard Henderson <richard.hender...@linaro.org> r~
