linux-kernel - Re: [PATCH v4 0/8] Remove dependency on congestion

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Date:   Fri, 22 Oct 2021 22:26:30 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Mel Gorman" <mgorman@...hsingularity.net>
Cc:     "Andrew Morton" <akpm@...ux-foundation.org>,
        "Theodore Ts'o" <tytso@....edu>,
        "Andreas Dilger" <adilger.kernel@...ger.ca>,
        "Darrick J . Wong" <djwong@...nel.org>,
        "Matthew Wilcox" <willy@...radead.org>,
        "Michal Hocko" <mhocko@...e.com>,
        "Dave Chinner" <david@...morbit.com>,
        "Rik van Riel" <riel@...riel.com>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Johannes Weiner" <hannes@...xchg.org>,
        "Jonathan Corbet" <corbet@....net>,
        "Linux-MM" <linux-mm@...ck.org>,
        "Linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
        "LKML" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 0/8] Remove dependency on congestion_wait in mm/

On Fri, 22 Oct 2021, Mel Gorman wrote:
> On Fri, Oct 22, 2021 at 12:15:10PM +1100, NeilBrown wrote:
> 
> > In general, I still don't like the use of wake_up_all(), though it won't
> > cause incorrect behaviour.
> > 
> 
> Removing wake_up_all would be tricky.

I think there is a misunderstanding.  Removing wake_up_all() is as
simple as
   s/wake_up_all/wake_up/

If you used prepare_to_wait_exclusive(), then wake_up() would only wake
one waiter, while wake_up_all() would wake all of them.
As you use prepare_to_wait(), wake_up() will wake all waiters - as will
wake_up_all(). 

When I see "wake_up_all()" I assume it is an exclusive wait, and that
for some reason this particular wake_up needs to wake up all waiters.
That is not the case here.

I suspect it would be clearer if "wake_up" always woke everything, and
"wake_up_one" was the special case - but unfortunately that isn't what
we have.

There are other non-exclusive waiters which use wake_up_all(), but the
vast majority of wakeups use wake_up(), and most of those are for
non-exclusive waiters.

Thanks,
NeilBrown