Commit-ID: e9d8c61557687b7126101e9550bdf243223f0d8f
Gitweb: https://git.kernel.org/tip/e9d8c61557687b7126101e9550bdf243223f0d8f
Author: Rik van Riel <riel@xxxxxxxxxxx>
AuthorDate: Mon, 16 Jul 2018 15:03:37 -0400
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 17 Jul 2018 09:35:34 +0200
x86/mm/tlb: Skip atomic operations for 'init_mm' in switch_mm_irqs_off()
Song Liu 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.
Reported-and-tested-by: Song Liu <songliubraving@xxxxxx>
Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
Acked-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: efault@xxxxxx
Cc: kernel-team@xxxxxx
Cc: luto@xxxxxxxxxx
Link: http://lkml.kernel.org/r/20180716190337.26133-8-riel@xxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/mm/tlb.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 493559cae2d5..f086195f644c 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -310,15 +310,22 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
sync_current_stack_to_mm(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));
+ /*
+ * Stop remote flushes for the previous mm.
+ * Skip kernel threads; we never send init_mm TLB flushing IPIs,
+ * but the bitmap manipulation can cause cache line contention.
+ */
+ 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);
--
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