On 08/02/2013 05:12 AM, 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)
The cacheline contention on the mm->cpu_vm_mask_va cpulist bitmask skyrockets on numa systems as soon as the number of threads exceeds the available cpus - and it's all from the locked bts instruction in cpumask_test_and_set_cpu(). The OMP version of Linpack uses clone() with the CLONE_VM flag set, so all the clones are tugging at the same mm->cpu_vm_mask_va.
With Rik's patch, the number of accesses tomm->cpu_vm_mask_va plummets.
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
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