linux-kernel - Re: [RFC PATCH v3 13/15] context_tracking,x86: Add infrastructure to defer kernel TLBI

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Message-ID: <20241209121249.GN35539@noisy.programming.kicks-ass.net>
Date: Mon, 9 Dec 2024 13:12:49 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Valentin Schneider <vschneid@...hat.com>
Cc: Petr Tesarik <ptesarik@...e.com>, Dave Hansen <dave.hansen@...el.com>,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
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Subject: Re: [RFC PATCH v3 13/15] context_tracking,x86: Add infrastructure to
 defer kernel TLBI

On Mon, Dec 09, 2024 at 01:04:43PM +0100, Valentin Schneider wrote:

> > But I wonder what exactly was the original scenario encountered by
> > Valentin. I mean, if TLB entry invalidations were necessary to sync
> > changes to kernel text after flipping a static branch, then it might be
> > less overhead to make a list of affected pages and call INVLPG on them.

No; TLB is not involved with text patching (on x86).

> > Valentin, do you happen to know?
> 
> So from my experimentation (hackbench + kernel compilation on housekeeping
> CPUs, dummy while(1) userspace loop on isolated CPUs), the TLB flushes only
> occurred from vunmap() - mainly from all the hackbench threads coming and
> going.

Right, we have virtually mapped stacks.