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

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Message-ID: <1531698583.26425.3.camel@surriel.com>
Date:   Sun, 15 Jul 2018 19:49:43 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, luto@...nel.org,
        dave.hansen@...el.com, kernel-team@...com, efault@....de,
        tglx@...utronix.de, songliubraving@...com, hpa@...or.com
Subject: Re: [PATCH 7/7] x86,switch_mm: skip atomic operations for init_mm

On Mon, 2018-07-16 at 01:04 +0200, Ingo Molnar wrote:
> * Rik van Riel <riel@...riel.com> wrote:
> 
> > +		/*
> > +		 * 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_pr
> > ev)));
> > +			cpumask_clear_cpu(cpu,
> > mm_cpumask(real_prev));
> 
> BTW., could this optimization be (safely) extended to all (or most)
> !task->mm 
> kernel threads?
> 
> In particular softirq and threaded irq handlers could benefit greatly
> I suspect in 
> certain networking intense workloads that happen to active them.

Yes, it could.

Are there kernel threads that use something other than
init_mm today?

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