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

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Message-ID: <20250711-ermangelung-darmentleerung-394cebde2708@brauner>
Date: Fri, 11 Jul 2025 11:44:28 +0200
From: Christian Brauner <brauner@...nel.org>
To: Nam Cao <namcao@...utronix.de>
Cc: Xi Ruoyao <xry111@...111.site>, 
	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, 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);
> +	}
> +
>  	__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.