linux-kernel - Re: [PATCH] fs/dcache.c: re-add cond_resched() in shrink_dcache

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Date:   Sat, 14 Apr 2018 21:58:46 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nikolay Borisov <nborisov@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Khazhismel Kumykov <khazhy@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Rientjes <rientjes@...gle.com>,
        Goldwyn Rodrigues <rgoldwyn@...e.de>,
        Jeff Mahoney <jeffm@...e.com>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH] fs/dcache.c: re-add cond_resched() in
 shrink_dcache_parent()

On Sat, Apr 14, 2018 at 09:36:23AM -0700, Linus Torvalds wrote:
> But it does *not* make sense for the case where we've hit a dentry
> that is already on the shrink list. Sure, we'll continue to gather all
> the other dentries, but if there is concurrent shrinking, shouldn't we
> give up the CPU more eagerly - *particularly* if somebody else is
> waiting (it might be the other process that actually gets rid of the
> shrinking dentries!)?
> 
> So my gut feel is that we should at least try doing something like
> this in select_collect():
> 
> -       if (!list_empty(&data->dispose))
> +       if (data->found)
>                 ret = need_resched() ? D_WALK_QUIT : D_WALK_NORETRY;
> 
> because even if we haven't actually been able to shrink something, if
> we hit an already shrinking entry we should probably at least not do
> the "retry for rename". And if we actually are going to reschedule, we
> might as well start from the beginning.
> 
> I realize that *this* thread might not be making any actual progress
> (because it didn't find any dentries to shrink), but since it did find
> _a_ dentry that is being shrunk, we know the operation itself - on a
> bigger scale - is making progress.
> 
> Hmm?

That breaks d_invalidate(), unfortunately.  Look at the termination
conditions in the loop there...