linux-kernel - Re: [patch V3 3/8] task isolation: sync vmstats on return to userspace

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Date:   Fri, 10 Sep 2021 15:49:42 +0200
From:   nsaenzju@...hat.com
To:     Marcelo Tosatti <mtosatti@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Nitesh Lal <nilal@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alex Belits <abelits@...its.com>, Peter Xu <peterx@...hat.com>
Subject: Re: [patch V3 3/8] task isolation: sync vmstats on return to
 userspace

Hi Marcelo,

On Tue, 2021-08-24 at 12:24 -0300, Marcelo Tosatti wrote:
> The logic to disable vmstat worker thread, when entering
> nohz full, does not cover all scenarios. For example, it is possible
> for the following to happen:
> 
> 1) enter nohz_full, which calls refresh_cpu_vm_stats, syncing the stats.
> 2) app runs mlock, which increases counters for mlock'ed pages.
> 3) start -RT loop
> 
> Since refresh_cpu_vm_stats from nohz_full logic can happen _before_
> the mlock, vmstat shepherd can restart vmstat worker thread on
> the CPU in question.
> 
> To fix this, use the task isolation prctl interface to quiesce 
> deferred actions when returning to userspace.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> 
> ---

[...]

> Index: linux-2.6/kernel/entry/common.c
> ===================================================================
> --- linux-2.6.orig/kernel/entry/common.c
> +++ linux-2.6/kernel/entry/common.c
> @@ -6,6 +6,7 @@
>  #include <linux/livepatch.h>
>  #include <linux/audit.h>
>  #include <linux/tick.h>
> +#include <linux/task_isolation.h>
>  
>  #include "common.h"
>  
> @@ -287,6 +288,7 @@ static void syscall_exit_to_user_mode_pr
>  static __always_inline void __syscall_exit_to_user_mode_work(struct pt_regs *regs)
>  {
>  	syscall_exit_to_user_mode_prepare(regs);
> +	isolation_exit_to_user_mode_prepare();

Are safe from migration at this stage?

-- 
Nicolás Sáenz