path: root/arch/x86
diff options
authorRik van Riel <riel@redhat.com>2013-07-31 22:14:21 -0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-09-26 17:18:14 -0700
commitc18c0f9da9d97ea6710804971b981456a40cd01a (patch)
treed51074be11b816a5cb20e821474160fb43ed9928 /arch/x86
parent8ceb02f11454cf5102bfede5a6e112abe3990bb2 (diff)
sched/x86: Optimize switch_mm() for multi-threaded workloads
commit 8f898fbbe5ee5e20a77c4074472a1fd088dc47d1 upstream. 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> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'arch/x86')
1 files changed, 13 insertions, 7 deletions
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index cdbf3677610..be12c534fd5 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -45,22 +45,28 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
/* Re-load page tables */
- /* stop flush ipis for the previous mm */
+ /* Stop flush ipis for the previous mm */
cpumask_clear_cpu(cpu, mm_cpumask(prev));
- /*
- * load the LDT, if the LDT is different:
- */
+ /* Load the LDT, if the LDT is different: */
if (unlikely(prev->context.ldt != next->context.ldt))
- else {
+ 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))) {
- /* We were in lazy tlb mode and leave_mm disabled
+ 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));
+ /*
+ * 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.