* tip-bot for Rik van Riel <tipbot@xxxxxxxxx> wrote: > Commit-ID: 8f898fbbe5ee5e20a77c4074472a1fd088dc47d1 > Gitweb: http://git.kernel.org/tip/8f898fbbe5ee5e20a77c4074472a1fd088dc47d1 > Author: Rik van Riel <riel@xxxxxxxxxx> > AuthorDate: Wed, 31 Jul 2013 22:14:21 -0400 > Committer: Ingo Molnar <mingo@xxxxxxxxxx> > CommitDate: Thu, 1 Aug 2013 09:10:26 +0200 > > sched/x86: Optimize switch_mm() for multi-threaded workloads > > Dick Fowles, Don Zickus and Joe Mario have been working on > improvements to perf, and noticed heavy cache line contention > on the mm_cpumask, running linpack on a 60 core / 120 thread > system. > > The cause turned out to be unnecessary atomic accesses to the > mm_cpumask. When in lazy TLB mode, the CPU is only removed from > the mm_cpumask if there is a TLB flush event. > > Most of the time, no such TLB flush happens, and the kernel > skips the TLB reload. It can also skip the atomic memory > set & test. > > Here is a summary of Joe's test results: > > * The __schedule function dropped from 24% of all program cycles down > to 5.5%. > > * The cacheline contention/hotness for accesses to that bitmask went > from being the 1st/2nd hottest - down to the 84th hottest (0.3% of > all shared misses which is now quite cold) > > * The average load latency for the bit-test-n-set instruction in > __schedule dropped from 10k-15k cycles down to an average of 600 cycles. > > * The linpack program results improved from 133 GFlops to 144 GFlops. > Peak GFlops rose from 133 to 153. > > Reported-by: Don Zickus <dzickus@xxxxxxxxxx> > Reported-by: Joe Mario <jmario@xxxxxxxxxx> > Tested-by: Joe Mario <jmario@xxxxxxxxxx> > Signed-off-by: Rik van Riel <riel@xxxxxxxxxx> > Reviewed-by: Paul Turner <pjt@xxxxxxxxxx> > Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Link: http://lkml.kernel.org/r/20130731221421.616d3d20@xxxxxxxxxxxxxxxxxxxxx > [ Made the comments consistent around the modified code. ] > Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> > + else { > this_cpu_write(cpu_tlbstate.state, TLBSTATE_OK); > BUG_ON(this_cpu_read(cpu_tlbstate.active_mm) != next); > > - if (!cpumask_test_and_set_cpu(cpu, mm_cpumask(next))) { > + if (!cpumask_test_cpu(cpu, mm_cpumask(next))) { > + /* > + * On established mms, the mm_cpumask is only changed > + * from irq context, from ptep_clear_flush() while in > + * lazy tlb mode, and here. Irqs are blocked during > + * schedule, protecting us from simultaneous changes. > + */ > + cpumask_set_cpu(cpu, mm_cpumask(next)); Note, I marked this for v3.12 with no -stable backport tag as it's not a regression fix. Nevertheless if it's a real issue in production (and +20% of linpack performance is certainly significant) feel free to forward it to -stable once this hits Linus's tree in the v3.12 merge window - by that time the patch will be reasonably well tested and it's a relatively simple change. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html