linux-kernel - Re: [PATCH-v2 1/3] percpu_ida: Make percpu_ida_alloc + callers accept task state bitmask

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Date:	Thu, 23 Jan 2014 16:43:12 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kent Overstreet <kmo@...erainc.com>
Cc:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	target-devel <target-devel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH-v2 1/3] percpu_ida: Make percpu_ida_alloc + callers
 accept task state bitmask

On Thu, Jan 23, 2014 at 05:55:39AM -0800, Kent Overstreet wrote:
> On Thu, Jan 23, 2014 at 02:50:03PM +0100, Peter Zijlstra wrote:
> > On Thu, Jan 23, 2014 at 05:28:29AM -0800, Kent Overstreet wrote:
> > > pool->lock is also going to be fairly badly contended in the worst case,
> > > and that can get real bad real fast... now that I think about it we
> > > probably want to avoid the __alloc_global_tag() double call just because
> > > of that, pool->lock is going to be quite a bit more contended than the
> > > waitlist lock just because fo the amount of work done under it.
> > 
> > OK, how about this then.. Not quite at pretty though
> 
> Heh, I suppose that is a solution. Let me screw around to see what I can
> come up with tomorrow, but if I can't come up with anything I like I'm
> not opposed to this.

Please also consider the below patch.

---
Subject: percpu-ida: Reduce IRQ held duration

Its bad manners to hold IRQs disabled over a full cpumask iteration.
Change it so that it disables the IRQs only where strictly required to
avoid lock inversion issues.

Signed-off-by: Peter Zijlstra <peterz@...radead.org>
---
--- a/lib/percpu_ida.c
+++ b/lib/percpu_ida.c
@@ -342,33 +342,31 @@ EXPORT_SYMBOL_GPL(__percpu_ida_init);
 int percpu_ida_for_each_free(struct percpu_ida *pool, percpu_ida_cb fn,
 	void *data)
 {
-	unsigned long flags;
 	struct percpu_ida_cpu *remote;
-	unsigned cpu, i, err = 0;
+	unsigned long flags;
+	int cpu, i, err = 0;
 
-	local_irq_save(flags);
 	for_each_possible_cpu(cpu) {
 		remote = per_cpu_ptr(pool->tag_cpu, cpu);
-		spin_lock(&remote->lock);
+		spin_lock_irqsave(&remote->lock, flags);
 		for (i = 0; i < remote->nr_free; i++) {
 			err = fn(remote->freelist[i], data);
 			if (err)
 				break;
 		}
-		spin_unlock(&remote->lock);
+		spin_unlock_irqrestore(&remote->lock, flags);
 		if (err)
-			goto out;
+			return err;
 	}
 
-	spin_lock(&pool->lock);
+	spin_lock_irqsave(&pool->lock, flags);
 	for (i = 0; i < pool->nr_free; i++) {
 		err = fn(pool->freelist[i], data);
 		if (err)
 			break;
 	}
-	spin_unlock(&pool->lock);
-out:
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&pool->lock, flags);
+
 	return err;
 }
 EXPORT_SYMBOL_GPL(percpu_ida_for_each_free);
--
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