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Date:   Mon, 18 May 2020 10:30:20 +1000
From:   Dave Chinner <david@fromorbit.com>
To:     Brian Foster <bfoster@redhat.com>
Cc:     linux-xfs@vger.kernel.org
Subject: Re: [PATCH 4/5] [RFC] xfs: per-cpu CIL lists
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On Fri, May 15, 2020 at 01:26:49PM -0400, Brian Foster wrote:
> Ok, that might prohibit using a bitop in the commit path. I'd still like
> to see actual numbers on that, though, just to see where on the spectrum
> it lands. I'm also wondering if the fast path logic mentioned above
> could be implemented like the following (using bitops instead of the
> spinlock):
> 
> 	if (test_bit(XLOG_CIL_EMPTY, ...) &&
> 	    test_and_clear_bit(XLOG_CIL_EMPTY, ...)) {
> 		<steal CIL res>
> 	}
> 
> That type of pattern seems to be used in at least a few other places in
> the kernel (e.g. filemap_check_errors(), wb_start_writeback(),
> __blk_mq_tag_busy()), presumably for similar reasons.

Ok, that seems reasonable given that there is other code using the
same pattern to avoid atomic ops. Overhead will be no different to
the test/lock/retest pattern I've been using...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com