linux-kernel - Re: [PATCH 01/12] x86/mm: make MMU_GATHER_RCU_TABLE

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Message-ID: <20250104162713.GA22803@noisy.programming.kicks-ass.net>
Date: Sat, 4 Jan 2025 17:27:13 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Rik van Riel <riel@...riel.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	dave.hansen@...ux.intel.com, luto@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, akpm@...ux-foundation.org,
	nadav.amit@...il.com, zhengqi.arch@...edance.com,
	linux-mm@...ck.org
Subject: Re: [PATCH 01/12] x86/mm: make MMU_GATHER_RCU_TABLE_FREE
 unconditional

On Fri, Jan 03, 2025 at 01:18:43PM +0100, Borislav Petkov wrote:
> On Thu, Jan 02, 2025 at 08:56:09PM +0100, Peter Zijlstra wrote:
> > Well, I've already answered why we need this in the previous thread but
> > it wasn't preserved :-(
> 
> ... and this needs to be part of the commit message. And there's a similar
> comment over tlb_remove_table_smp_sync() in mm/mmu_gather.c which pretty much
> explains the same thing.
> 
> > Currently GUP-fast serializes against table-free by disabling
> > interrupts, which in turn holds of the TLBI-IPIs.
> > 
> > Since you're going to be doing broadcast TLBI -- without IPIs, this no
> > longer works and we need other means of serializing GUP-fast vs
> > table-free.
> > 
> > MMU_GATHER_RCU_TABLE_FREE is that means.
> > 
> > So where previously paravirt implementations of tlb_flush_multi might
> > require this (because of virt optimizations that avoided the TLBI-IPI),
> > this broadcast invalidate now very much requires this for native.
> 
> Right, so this begs the question: we probably should do this dynamically only
> on TLBI systems - not on everything native - due to the overhead of this
> batching - I'm looking at tlb_remove_table().
> 
> Or should we make this unconditional on all native because we don't care about
> the overhead and would like to have simpler code. I mean, disabling IRQs vs
> batching and allocating memory...?

The disabling IRQs on the GUP-fast side stays, it acts as a
RCU-read-side section -- also mmu_gather reverts to sending IPIs if it
runs out of memory (extremely rare).

I don't think there is measurable overhead from doing the separate table
batching, but I'm sure the robots will tell us.