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From: Matthew Wilcox <>
To: Jens Axboe <>
Cc: Andrew Morton <>,
	Sebastian Andrzej Siewior <>,,,
	Nicholas Bellinger <>,
	Shaohua Li <>,
	Kent Overstreet <>
Subject: Re: [PATCH] percpu_ida: Use _irqsave() instead of local_irq_save() + spin_lock
Date: Mon, 7 May 2018 06:47:31 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Sat, May 05, 2018 at 08:52:02AM -0700, Matthew Wilcox wrote:
> init and destroy seem to map to sbitmap_queue_init_node and
> sbitmap_queue_free.  percpu_ida_free maps to sbitmap_queue_clear.


void sbitmap_queue_clear(struct sbitmap_queue *sbq, unsigned int nr,
                         unsigned int cpu)
        sbitmap_clear_bit_unlock(&sbq->sb, nr);
        if (likely(!sbq->round_robin && nr < sbq->sb.depth))
                *per_cpu_ptr(sbq->alloc_hint, cpu) = nr;

If we free a tag on a CPU other than the one it's allocated on, that seems
like it's going to guarantee a cacheline pingpong.  Is the alloc_hint
really that valuable?  I'd be tempted to maintain the alloc_hint (if it's
at all valuable) as being just a hint for which word to look at first,
and only update it on allocation, rather than updating it on free.
Then we can drop the 'cpu' argument to sbitmap_queue_clear(), which
would help this conversion because the percpu_ida users don't know what
CPU their tag was allocated on.

  reply	other threads:[~2018-05-07 13:47 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-04 15:32 Sebastian Andrzej Siewior
2018-05-04 23:22 ` Andrew Morton
2018-05-05  3:51   ` Matthew Wilcox
2018-05-05 14:10     ` Jens Axboe
2018-05-05 14:42       ` Jens Axboe
2018-05-05 15:52       ` Matthew Wilcox
2018-05-07 13:47         ` Matthew Wilcox [this message]
2018-05-07 21:34           ` Jens Axboe

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