Re: [tip:sched/core] sched/x86: Optimize switch_mm() for multi-threaded workloads

[Ingo Molnar <mingo@kernel.org>]

* tip-bot for Rik van Riel <tipbot@zytor.com> wrote:

> Commit-ID:  8f898fbbe5ee5e20a77c4074472a1fd088dc47d1
> Gitweb:     http://git.kernel.org/tip/8f898fbbe5ee5e20a77c4074472a1fd088dc47d1
> Author:     Rik van Riel <riel@redhat.com>
> AuthorDate: Wed, 31 Jul 2013 22:14:21 -0400
> Committer:  Ingo Molnar <mingo@kernel.org>
> 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@redhat.com>
> Reported-by: Joe Mario <jmario@redhat.com>
> Tested-by: Joe Mario <jmario@redhat.com>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> Reviewed-by: Paul Turner <pjt@google.com>
> Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
> Link: http://lkml.kernel.org/r/20130731221421.616d3d20@annuminas.surriel.com
> [ Made the comments consistent around the modified code. ]
> Signed-off-by: Ingo Molnar <mingo@kernel.org>

> +	  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