linux-kernel - Re: [PATCH v2] eventpoll: fix missing wakeup for ovflist in ep_poll

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Date:   Sat, 25 Apr 2020 12:17:08 -0400
From:   Jason Baron <jbaron@...mai.com>
To:     Khazhismel Kumykov <khazhy@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Roman Penyaev <rpenyaev@...e.de>,
        Alexander Viro <viro@...iv.linux.org.uk>, Heiher <r@....cc>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] eventpoll: fix missing wakeup for ovflist in
 ep_poll_callback



On 4/24/20 3:00 PM, Khazhismel Kumykov wrote:
> In the event that we add to ovflist, before 339ddb53d373 we would be
> woken up by ep_scan_ready_list, and did no wakeup in ep_poll_callback.
> With that wakeup removed, if we add to ovflist here, we may never wake
> up. Rather than adding back the ep_scan_ready_list wakeup - which was
> resulting in unnecessary wakeups, trigger a wake-up in ep_poll_callback.

I'm just curious which 'wakeup' we are talking about here? There is:
wake_up(&ep->wq) for the 'ep' and then there is the nested one via:
ep_poll_safewake(ep, epi). It seems to me that its only about the later
one being missing not both? Is your workload using nested epoll?

If so, it might make sense to just do the later, since the point of
the original patch was to minimize unnecessary wakeups.

Thanks,

-Jason

> 
> We noticed that one of our workloads was missing wakeups starting with
> 339ddb53d373 and upon manual inspection, this wakeup seemed missing to
> me. With this patch added, we no longer see missing wakeups. I haven't
> yet tried to make a small reproducer, but the existing kselftests in
> filesystem/epoll passed for me with this patch.
> 
> Fixes: 339ddb53d373 ("fs/epoll: remove unnecessary wakeups of nested epoll")
> 
> Signed-off-by: Khazhismel Kumykov <khazhy@...gle.com>
> Reviewed-by: Roman Penyaev <rpenyaev@...e.de>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: Heiher <r@....cc>
> Cc: Jason Baron <jbaron@...mai.com>
> Cc: <stable@...r.kernel.org>
> ---
> v2: use if/elif instead of goto + cleanup suggested by Roman
>  fs/eventpoll.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index 8c596641a72b..d6ba0e52439b 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -1171,6 +1171,10 @@ static inline bool chain_epi_lockless(struct epitem *epi)
>  {
>  	struct eventpoll *ep = epi->ep;
>  
> +	/* Fast preliminary check */
> +	if (epi->next != EP_UNACTIVE_PTR)
> +		return false;
> +
>  	/* Check that the same epi has not been just chained from another CPU */
>  	if (cmpxchg(&epi->next, EP_UNACTIVE_PTR, NULL) != EP_UNACTIVE_PTR)
>  		return false;
> @@ -1237,16 +1241,12 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
>  	 * chained in ep->ovflist and requeued later on.
>  	 */
>  	if (READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR) {
> -		if (epi->next == EP_UNACTIVE_PTR &&
> -		    chain_epi_lockless(epi))
> +		if (chain_epi_lockless(epi))
> +			ep_pm_stay_awake_rcu(epi);
> +	} else if (!ep_is_linked(epi)) {
> +		/* In the usual case, add event to ready list. */
> +		if (list_add_tail_lockless(&epi->rdllink, &ep->rdllist))
>  			ep_pm_stay_awake_rcu(epi);
> -		goto out_unlock;
> -	}
> -
> -	/* If this file is already in the ready list we exit soon */
> -	if (!ep_is_linked(epi) &&
> -	    list_add_tail_lockless(&epi->rdllink, &ep->rdllist)) {
> -		ep_pm_stay_awake_rcu(epi);
>  	}
>  
>  	/*
>