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

From: Joe Mario
Date: Fri Aug 02 2013 - 08:44:27 EST

On 08/02/2013 05:12 AM, Ingo Molnar wrote:
* tip-bot for Rik van Riel <tipbot@xxxxxxxxx> wrote:

Commit-ID: 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

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>
[ 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.



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