linux-kernel - Re: [PATCH] percpu_ida: Handle out-of-tags gracefully

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Message-ID: <20140311204826.GA30105@dhcp-26-207.brq.redhat.com>
Date:	Tue, 11 Mar 2014 21:48:28 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Bart Van Assche <bvanassche@....org>
Cc:	Jens Axboe <axboe@...nel.dk>, Kent Overstreet <kmo@...erainc.com>,
	Shaohua Li <shli@...nel.org>, Christoph Hellwig <hch@....de>,
	Mike Christie <michaelc@...wisc.edu>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] percpu_ida: Handle out-of-tags gracefully

On Tue, Mar 11, 2014 at 07:10:18PM +0100, Bart Van Assche wrote:
> > I assume the BUG() above hits? If so, I am failing to understand how
> > the code gets here. Mind elaborate?
> 
> Hello Alexander,
> 
> You are correct, the BUG() mentioned in the call stack in the
> description of this patch does indeed correspond with the BUG()
> statement in the above code. That BUG() was encountered while testing
> the scsi-mq patch series with a workload with a large queue depth. I
> think the fact that I hit that BUG() statement means that my workload
> was queueing requests faster than these were processed by the SCSI LLD
> and hence that percpu_ida_alloc() ran out of tags.

Function steal_tags() is entered with disabled interrupts and
pool->lock taken. Then the 'for' cycle enters/loops while 'cpus_have_tags'
is not zero. Which means we can not end up with no set bits at all -
and that is the reason why BUG() is (legitimately) placed there.

> Bart.

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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