Song noticed switch_mm_irqs_off taking a lot of CPU time in recent kernels,using 1.8% of a 48 CPU system during a netperf to localhost run. Digging into the profile, we noticed that cpumask_clear_cpu and cpumask_set_cpu together take about half of the CPU time taken by switch_mm_irqs_off.
However, the CPUs running netperf end up switching back and forth between netperf and the idle task, which does not require changes to the mm_cpumask. Furthermore, the init_mm cpumask ends up being the most heavily contended one in the system. Simply skipping changes to mm_cpumask(&init_mm) reduces overhead. Signed-off-by: Rik van Riel <[email protected]> Reported-and-tested-by: Song Liu <[email protected]> --- arch/x86/mm/tlb.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index 5a01fcb22a7e..b55e6b7df7c9 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -311,14 +311,17 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, } /* Stop remote flushes for the previous mm */ - VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu, mm_cpumask(real_prev)) && - real_prev != &init_mm); - cpumask_clear_cpu(cpu, mm_cpumask(real_prev)); + if (real_prev != &init_mm) { + VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu, + mm_cpumask(real_prev))); + cpumask_clear_cpu(cpu, mm_cpumask(real_prev)); + } /* * Start remote flushes and then read tlb_gen. */ - cpumask_set_cpu(cpu, mm_cpumask(next)); + if (next != &init_mm) + cpumask_set_cpu(cpu, mm_cpumask(next)); next_tlb_gen = atomic64_read(&next->context.tlb_gen); choose_new_asid(next, next_tlb_gen, &new_asid, &need_flush); -- 2.14.4
