linux-kernel - Re: [PATCH 5/6] sched/isolation: Introduce isolated task work

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Message-ID: <Z_mQ4-7BeCCJGSIN@pavilion.home>
Date: Sat, 12 Apr 2025 00:00:03 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Michal Hocko <mhocko@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org
Subject: Re: [PATCH 5/6] sched/isolation: Introduce isolated task work

Le Fri, Apr 11, 2025 at 12:25:56PM +0200, Oleg Nesterov a écrit :
> I know nothing about this code so I can't review, but let me
> ask anyway...
> 
> On 04/10, Frederic Weisbecker wrote:
> >
> > +int __isolated_task_work_queue(void)
> > +{
> > +	unsigned long flags;
> > +	int ret;
> > +
> > +	if (current->flags & PF_KTHREAD)
> > +		return -EINVAL;
> 
> What about PF_USER_WORKER's ? IIUC, these (in fact kernel) threads
> never return to userspace and never call task_work_run().

Ah good catch! (though I'm having a hard time finding out what this is
about)...

> 
> Or PF_IO_WORKER's, they too run only in kernel mode... But iirc they
> do call task_work_run().

At least I see a lot of task_work usage in io_uring, and there are some
explicit calls to task_work_run() there...

> 
> > +	local_irq_save(flags);
> > +	if (task_work_queued(&current->nohz_full_work)) {
> > +		ret = 0;
> > +		goto out;
> > +	}
> > +
> > +	ret = task_work_add(current, &current->nohz_full_work, TWA_RESUME);
> > +out:
> > +	local_irq_restore(flags);
> > +	return ret;
> 
> Hmm, why not
> 
> 	local_irq_save(flags);
> 	if (task_work_queued(...))
> 		ret = 0;
> 	else
> 		ret = task_work_add(...);

Hehe, yes indeed!

Thanks!

-- 
Frederic Weisbecker
SUSE Labs