On Fri, Aug 02, 2013 at 11:12:47AM +0200, Ingo Molnar wrote: > * 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. I'll watch for this as well and try to remember to pick it up for -stable once it hits Linus's tree, as those type of benchmark improvements are good to have in stable releases. thanks, greg k-h -- 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