linux-kernel - Re: [PATCH v3] eventpoll: Fix priority inversion problem

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Message-ID: <6856a981f0505233726af0301a1fb1331acdce1c.camel@xry111.site>
Date: Fri, 11 Jul 2025 17:48:56 +0800
From: Xi Ruoyao <xry111@...111.site>
To: Christian Brauner <brauner@...nel.org>, Nam Cao <namcao@...utronix.de>
Cc: Frederic Weisbecker <frederic@...nel.org>, Valentin Schneider	
 <vschneid@...hat.com>, Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara	
 <jack@...e.cz>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, John
 Ogness	 <john.ogness@...utronix.de>, Clark Williams <clrkwllms@...nel.org>,
 Steven Rostedt <rostedt@...dmis.org>, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, 	linux-rt-devel@...ts.linux.dev,
 linux-rt-users@...r.kernel.org, Joe Damato	 <jdamato@...tly.com>, Martin
 Karsten <mkarsten@...terloo.ca>, Jens Axboe	 <axboe@...nel.dk>
Subject: Re: [PATCH v3] eventpoll: Fix priority inversion problem

On Fri, 2025-07-11 at 11:44 +0200, Christian Brauner wrote:
> On Fri, Jul 11, 2025 at 07:02:17AM +0200, Nam Cao wrote:
> > On Thu, Jul 10, 2025 at 05:47:57PM +0800, Xi Ruoyao wrote:
> > > It didn't work :(.
> > 
> > Argh :(
> > 
> > Another possibility is that you are running into event starvation problem.
> > 
> > Can you give the below patch a try? It is not the real fix, the patch hurts
> > performance badly. But if starvation is really your problem, it should
> > ameliorate the situation:
> > 
> > diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> > index 895256cd2786..0dcf8e18de0d 100644
> > --- a/fs/eventpoll.c
> > +++ b/fs/eventpoll.c
> > @@ -1764,6 +1764,8 @@ static int ep_send_events(struct eventpoll *ep,
> >  		__llist_add(n, &txlist);
> >  	}
> >  
> > +	struct llist_node *shuffle = llist_del_all(&ep->rdllist);
> > +
> >  	llist_for_each_entry_safe(epi, tmp, txlist.first, rdllink) {
> >  		init_llist_node(&epi->rdllink);
> >  
> > @@ -1778,6 +1780,13 @@ static int ep_send_events(struct eventpoll *ep,
> >  		}
> >  	}
> >  
> > +	if (shuffle) {
> > +		struct llist_node *last = shuffle;
> > +		while (last->next)
> > +			last = last->next;
> > +		llist_add_batch(shuffle, last, &ep->rdllist);
> > +	}
> > +

Sadly, still no luck.

> >  	__pm_relax(ep->ws);
> >  	mutex_unlock(&ep->mtx);
> >  
> 
> I think we should revert the fix so we have time to fix it properly
> during v6.17+. This patch was a bit too adventurous for a fix in the
> first place tbh.

I tried to understand the code but all the comments seem outdated (they
still mention the removed rwlock).  IMO we'd at least clean them up...
and maybe we can notice something erratic.

-- 
Xi Ruoyao <xry111@...111.site>