linux-kernel - Re: [PATCH v2 13/19] mm/migrate: Use xchg instead of spinlock

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Date:   Mon, 23 Jul 2018 16:04:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 13/19] mm/migrate: Use xchg instead of spinlock

On Mon, Jul 23, 2018 at 04:20:32AM -0700, Srikar Dronamraju wrote:
> > If you maybe write that like:
> > 
> > 	if (time_after(jiffies, next_window) &&
> > 	    xchg(&pgdat->numabalancing_migrate_nr_pages, 0UL)) {
> > 
> > 		do {
> > 			next_window += interval;
> > 		} while (unlikely(time_after(jiffies, next_window)));
> > 
> > 		WRITE_ONCE(pgdat->numabalancing_migrate_next_window, next_window);
> > 	}
> > 
> > Then you avoid an indent level and line-wrap, resulting imo easier to
> > read code.
> > 
> 
> Okay will do.

FWIW, that code seems to rely on @nr_pages != 0, otherwise you can have
the xchg fail even though time_after is true.

Probably not a problem, but it is a semantic change vs the spinlock.
Also, the spinlock thing could probably have changed to a trylock and
you'd have seens similar 'gains' I suppose.

Another difference vs the lock+unlock is that you lost the release
ordering. Again, probably not a big deal, but it does make the whole
things a little dodgy.