linux-kernel - Re: [PATCH V3 0/4] genirq/affinity: irq vector spread among online CPUs as far as possible

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Date:   Thu, 8 Mar 2018 21:34:45 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Artem Bityutskiy <dedekind1@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        Laurence Oberman <loberman@...hat.com>
Subject: Re: [PATCH V3 0/4] genirq/affinity: irq vector spread among online
 CPUs as far as possible

On Thu, Mar 08, 2018 at 03:18:33PM +0200, Artem Bityutskiy wrote:
> On Thu, 2018-03-08 at 18:53 +0800, Ming Lei wrote:
> > Hi,
> > 
> > This patchset tries to spread among online CPUs as far as possible, so
> > that we can avoid to allocate too less irq vectors with online CPUs
> > mapped.
> > 
> > For example, in a 8cores system, 4 cpu cores(4~7) are offline/non present,
> > on a device with 4 queues:
> > 
> > 1) before this patchset
> > 	irq 39, cpu list 0-2
> > 	irq 40, cpu list 3-4,6
> > 	irq 41, cpu list 5
> > 	irq 42, cpu list 7
> > 
> > 2) after this patchset
> > 	irq 39, cpu list 0,4
> > 	irq 40, cpu list 1,6
> > 	irq 41, cpu list 2,5
> > 	irq 42, cpu list 3,7
> > 
> > Without this patchset, only two vectors(39, 40) can be active, but there
> > can be 4 active irq vectors after applying this patchset.
> 
> Tested-by: Artem Bityutskiy <artem.bityutskiy@...el.com>
> Link: https://lkml.kernel.org/r/1519311270.2535.53.camel@intel.com

Hi Artem,

Thanks for your test!

> 
> Ming,
> 
> this patchset fixes the v4.16-rcX regression that I reported few weeks
> ago. I applied it and verified that Dell R640 server that I mentioned
> in the bug report boots up and the disk works.
> 
> So this is not just an improvement, it also includes a bugfix. 

Actually, it isn't a real fix, the real one is in the following two:

	0c20244d458e scsi: megaraid_sas: fix selection of reply queue
	ed6d043be8cd scsi: hpsa: fix selection of reply queue

This patchset can't guarantee that all IRQ vectors are assigned by one
online CPU, for example, in a quad-socket system, if only one processor
is present, then some of vectors are still assigned by all offline CPUs,
and it is a valid case, but still may cause io hang if drivers(hpsa, megaraid_sas)
select reply queue in current way.

Thanks,
Ming