linux-kernel - Re: [PATCH] x86,tlb: update mm

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Message-ID: <55ada30f76d544a3f700e57a01cb6f6f255581d5.camel@surriel.com>
Date: Fri, 08 Nov 2024 15:07:23 -0500
From: Rik van Riel <riel@...riel.com>
To: Dave Hansen <dave.hansen@...el.com>, Dave Hansen
	 <dave.hansen@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>,  Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
 <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,  x86@...nel.org, "H.
 Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org, 
 kernel-team@...a.com
Subject: Re: [PATCH] x86,tlb: update mm_cpumask lazily

On Fri, 2024-11-08 at 12:03 -0800, Dave Hansen wrote:
> On 11/8/24 11:31, Rik van Riel wrote:
> >  		/* Start receiving IPIs and then read tlb_gen (and
> > LAM below) */
> > -		if (next != &init_mm)
> > +		if (next != &init_mm && !cpumask_test_cpu(cpu,
> > mm_cpumask(next)))
> >  			cpumask_set_cpu(cpu, mm_cpumask(next));
> >  		next_tlb_gen = atomic64_read(&next-
> > >context.tlb_gen);
> 
> If we're worried about contention on mm_cpumask(), then this hunk
> makes
> sense independently of the lazy updating. We might want to take this
> hunk forward before we do the rest because this seems like a no-
> brainer.
> 
If we always clear the CPU in the mm_cpumask when prev != next,
wouldn't that result in that CPU's bit being clear (and needing
to be set) for next when prev != next?

What am I missing?

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