From: Jens Axboe <axboe@kernel.dk>
To: Christoph Hellwig <hch@infradead.org>
Cc: Steven Whitehouse <swhiteho@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: Preempt & smp_processor_id in __make_request
Date: Tue, 26 Jul 2011 15:05:19 +0200 [thread overview]
Message-ID: <4E2EBB8F.7000908@kernel.dk> (raw)
In-Reply-To: <20110726114330.GA12704@infradead.org>
On 2011-07-26 13:43, Christoph Hellwig wrote:
> On Tue, Jul 26, 2011 at 10:32:06AM +0100, Steven Whitehouse wrote:
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index f8cb099..f925581 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -1283,7 +1283,7 @@ get_rq:
>>
>> if (test_bit(QUEUE_FLAG_SAME_COMP, &q->queue_flags) ||
>> bio_flagged(bio, BIO_CPU_AFFINE))
>> - req->cpu = smp_processor_id();
>> + req->cpu = raw_smp_processor_id();
>>
>> plug = current->plug;
>> if (plug) {
>>
>> However this fixes the symptoms, rather than the cause, so I'm not at
>> all sure that this is the correct solution,
>
> It doesn't fix the symptoms - the warning is one of those 98% percent
> right types of warnings. In this case get_cpu/put_cpu doesn't buy you
> anthing - we want to stash away the cpu id that we submitted the I/O
> from, to compare it when we later process the I/O completion from
> a different context. With some PREEMPT options we might actually get
> rescheduled to a different CPU during the rest of this function, but
> as we submit the plugged requests at this point we'll still be correct.
>
> Even if wasn't we would already thrash the cache badly enough for this
> optimization not to matter in that case.
This was reported earlier today as well, and I suggested the same fix.
raw_smp_processor_id() is fine, since it's just a hint.
--
Jens Axboe
prev parent reply other threads:[~2011-07-26 13:05 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-07-26 9:32 Preempt & smp_processor_id in __make_request Steven Whitehouse
2011-07-26 10:56 ` Sergey Senozhatsky
2011-07-26 16:15 ` Williams, Dan J
2011-07-26 11:43 ` Christoph Hellwig
2011-07-26 13:05 ` Jens Axboe [this message]
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