linux-kernel - Re: [PATCh 0/3] x86,tlb: context switch optimizations

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Message-ID: <20241114113630.GP6497@noisy.programming.kicks-ass.net>
Date: Thu, 14 Nov 2024 12:36:30 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Rik van Riel <riel@...riel.com>, Borislav Petkov <bp@...en8.de>,
	linux-kernel@...r.kernel.org, dave.hansen@...ux.intel.com,
	luto@...nel.org, tglx@...utronix.de, mingo@...hat.com,
	x86@...nel.org, kernel-team@...a.com, hpa@...or.com,
	bigeasy@...utronix.de
Subject: Re: [PATCh 0/3] x86,tlb: context switch optimizations

On Thu, Nov 14, 2024 at 10:52:48AM +0100, Ingo Molnar wrote:
> 
> * Rik van Riel <riel@...riel.com> wrote:
> 
> > On Wed, 13 Nov 2024 10:55:50 +0100
> > Borislav Petkov <bp@...en8.de> wrote:
> > 
> > > On Fri, Nov 08, 2024 at 07:27:47PM -0500, Rik van Riel wrote:
> > > > While profiling switch_mm_irqs_off with several workloads,
> > > > it appears there are two hot spots that probably don't need
> > > > to be there.  
> > > 
> > > One of those three is causing the below here, zapping them from tip.
> > > 
> > 
> > TL;DR: __text_poke ends up sending IPIs with interrupts disabled.
> > 
> > > [    3.186469]  on_each_cpu_cond_mask+0x50/0x90
> > > [    3.186469]  flush_tlb_mm_range+0x1a8/0x1f0
> > > [    3.186469]  ? cpu_bugs_smt_update+0x14/0x1f0
> > > [    3.186469]  __text_poke+0x366/0x5d0
> > 
> > Here is an alternative to avoid __text_poke() from calling
> > on_each_cpu_cond_mask() with IRQs disabled:
> > 
> > ---8<---
> > From e872edeaad14c793036f290afc28000281e1b76a Mon Sep 17 00:00:00 2001
> > From: Rik van Riel <riel@...riel.com>
> > Date: Wed, 13 Nov 2024 09:51:16 -0500
> > Subject: [PATCH] x86/alternatives: defer poking_mm TLB flush to next use
> 
> I'd argue *both* of your patches improve the code, right?

No, please don't.