linux-kernel - [PATCH 7/7] x86,switch_mm: skip atomic operations for init

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Date:   Fri,  6 Jul 2018 17:56:58 -0400
From:   Rik van Riel <riel@...riel.com>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, luto@...nel.org, dave.hansen@...ux.intel.com,
        mingo@...nel.org, kernel-team@...com, tglx@...utronix.de,
        efault@....de, songliubraving@...com, hpa@...or.com,
        Rik van Riel <riel@...riel.com>
Subject: [PATCH 7/7] x86,switch_mm: skip atomic operations for init_mm

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@...riel.com>
Acked-by: Dave Hansen <dave.hansen@...el.com>
Reported-and-tested-by: Song Liu <songliubraving@...com>
---
 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 e7c6de7eb903..c8644aa12abd 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 the idle task; 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);
-- 
2.14.4