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Subject[PATCH] sched,x86: optimize switch_mm for multi-threaded workloads
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>
---
arch/x86/include/asm/mmu_context.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index cdbf367..987eb3d 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -59,11 +59,12 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
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))) {
/* We were in lazy tlb mode and leave_mm disabled
* tlb flush IPI delivery. We must reload CR3
* to make sure to use no freed page tables.
*/
+ cpumask_set_cpu(cpu, mm_cpumask(next));
load_cr3(next->pgd);
load_LDT_nolock(&next->context);
}

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