[PATCH] sched,x86: optimize switch_mm for multi-threaded workloads

[Rik van Riel]

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>
---
 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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