linux-kernel - Re: [PATCH v2 7/9] sched: define TIF_ALLOW

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Message-ID: <20231026084807.3d50aee9@gandalf.local.home>
Date:   Thu, 26 Oct 2023 08:48:07 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, jon.grimm@....com,
        bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com,
        Joel Fernandes <joel@...lfernandes.org>,
        Youssef Esmat <youssefesmat@...omium.org>,
        Vineeth Pillai <vineethrp@...gle.com>,
        Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

On Thu, 26 Oct 2023 16:50:16 +0900
Sergey Senozhatsky <senozhatsky@...omium.org> wrote:

> > The goal is to prevent a thread / vCPU being preempted while holding a lock
> > or resource that other threads / vCPUs will want. That is, prevent
> > contention, as that's usually the biggest issue with performance in user
> > space and VMs.  
> 
> I think some time ago we tried to check guest's preempt count on each vm-exit
> and we'd vm-enter if guest exited from a critical section (those that bump
> preempt count) so that it can hopefully finish whatever is was going to
> do and vmexit again. We didn't look into covering guest's RCU read-side
> critical sections.
> 
> Can you educate me, is your PoC significantly different from guest preempt
> count check?

No, it's probably very similar. Just the mechanism to allow it to run
longer may be different.

-- Steve