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

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Date:   Mon, 16 Jul 2018 14:37:04 -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 03:04 +0200, Ingo Molnar wrote:
> * Rik van Riel <riel@...riel.com> wrote:
> 
> > 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(rea
> > > > l_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?
> 
> Yeah, I think that's the typical case - so at minimum the comment
> should be fixed:
> 
>  > > > +	 * Skip the idle task; we never send init_mm TLB
> flushing IPIs,
> 
> and it should say 'skip kernel threads', right?

I will send a v6 that improves this comment, and has
the S-o-b thing you suggested for patch 1/7.

I think that addresses all the comments people had on
this patch series.

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