[PATCH 7/7] x86,switch_mm: skip atomic operations for init_mm

From: Rik van Riel
Date: Fri Jun 29 2018 - 10:29:55 EST


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 <riel@xxxxxxxxxxx>
Reported-and-tested-by: Song Liu <songliubraving@xxxxxx>
---
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