linux-kernel - Re: [PATCH v1 0/6] x86/resctrl: Avoid searching tasklist during mongrp

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Message-ID: <CALPaoCj1QVtVYcVV2FS1muL9Lo7f1MXuXaQQ+WKRi8h42aCCjw@mail.gmail.com>
Date: Thu, 11 Apr 2024 15:44:10 -0700
From: Peter Newman <peternewman@...gle.com>
To: babu.moger@....com
Cc: Fenghua Yu <fenghua.yu@...el.com>, Reinette Chatre <reinette.chatre@...el.com>, 
	James Morse <james.morse@....com>, Stephane Eranian <eranian@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>, 
	Uros Bizjak <ubizjak@...il.com>, Mike Rapoport <rppt@...nel.org>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Xin Li <xin3.li@...el.com>, 
	Shaopeng Tan <tan.shaopeng@...itsu.com>, 
	Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, Jens Axboe <axboe@...nel.dk>, 
	Christian Brauner <brauner@...nel.org>, Oleg Nesterov <oleg@...hat.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Tycho Andersen <tandersen@...flix.com>, 
	Nicholas Piggin <npiggin@...il.com>, Beau Belgrave <beaub@...ux.microsoft.com>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 0/6] x86/resctrl: Avoid searching tasklist during mongrp_reparent

Hi Babu,

On Thu, Apr 11, 2024 at 3:34 PM Moger, Babu <bmoger@....com> wrote:
>
> Hi Peter,
>
> On 3/25/2024 12:27 PM, Peter Newman wrote:
> > To avoid this issue, this series replaces the CLOSID and RMID values
> > cached in the task_struct with a pointer to the task's rdtgroup, through
> > which the current CLOSID and RMID can be obtained indirectly during a
> > context switch. Updating a group's ID then only requires the current
> > task to be switched back in on all CPUs. On server hosts with very large
> > thread counts, this is much less disruptive than making thread creation
> > globally unavailable. However, this is less desirable on CPU-isolated,
> > realtime workloads, so I am interested in suggestions on how to reach a
> > compromise for the two use cases.
>
> Before going this route, have you thought about your original solution
> where CONTROL_MON groups sharing the same CLOSID?
>
> [3] https://lore.kernel.org/lkml/CALPaoCj-zav4x6H3ffXo_O+RFan8Qb-uLy-DdtkaQTfuxY4a0w@mail.gmail.com
>
> May be it is less disruptive than changing the context switch code.. Thoughts?

If had I ever posted that series, it would have contained the very
same changes to the context switch code, because that solution causes
rdtgroup->closid fields to change on almost every schemata write.

-Peter