linux-kernel - Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping processes during pipe read/write

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Message-ID: <4tee2rwpqjmx7jj5poxxelv4sp2jyw6nuhpiwrlpv2lurgvpmz@3paxwuit47i6>
Date: Wed, 25 Dec 2024 17:04:46 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	WangYuli <wangyuli@...ontech.com>, viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz, 
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Subject: Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping
 processes during pipe read/write

On Wed, Dec 25, 2024 at 08:53:05AM -0500, Kent Overstreet wrote:
> On Wed, Dec 25, 2024 at 03:30:05PM +0200, Andy Shevchenko wrote:
> > Don't you think the Cc list is a bit overloaded?
> 
> Indeed, my mail server doesn't let me reply-all.
> 
> > On Wed, Dec 25, 2024 at 05:42:02PM +0800, WangYuli wrote:
> > > +config PIPE_SKIP_SLEEPER
> > > +	bool "Skip sleeping processes during pipe read/write"
> > > +	default n
> > 
> > 'n' is the default 'default', no need to have this line.
> 
> Actually, I'd say to skip the kconfig option for this. Kconfig options
> that affect the behaviour of core code increase our testing burden, and
> are another variable to account for when chasing down bugs, and the
> potential overhead looks negligable.
> 

I agree the behavior should not be guarded by an option. However,
because of how wq_has_sleeper is implemented (see below) I would argue
this needs to show how often locking can be avoided in real workloads.

The commit message does state this comes with a slowdown for cases which
can't avoid wakeups, but as is I thought the submitter just meant an
extra branch.

> Also, did you look at adding this optimization to wake_up()? No-op
> wakeups are very common, I think this has wider applicability.

I was going to suggest it myself, but then:

static inline bool wq_has_sleeper(struct wait_queue_head *wq_head)
{
        /*
         * We need to be sure we are in sync with the
         * add_wait_queue modifications to the wait queue.
         *
         * This memory barrier should be paired with one on the
         * waiting side.
         */
        smp_mb();
        return waitqueue_active(wq_head);
}

Which means this is in fact quite expensive.

Since wakeup is a lock + an interrupt trip, it would still be
cheaper single-threaded to "merely" suffer a full fence and for cases
where the queue is empty often enough this is definitely the right thing
to do.

On the other hand this executing when the queue is mostly *not* empty
would combat the point.

So unfortunately embedding this in wake_up is a no-go.