On Wed, Jul 31, 2013 at 2:43 PM, Rik van Riel <r...@redhat.com> wrote: > 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 <dzic...@redhat.com> > Reported-by: Joe Mario <jma...@redhat.com> > Tested-by: Joe Mario <jma...@redhat.com> > Signed-off-by: Rik van Riel <r...@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); > }
We're carrying the *exact* same patch for *exact* same reason. I've been meaning to send it out but wasn't sure of a good external workload for this. Reviewed-by: Paul Turner <p...@google.com> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/