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

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Date:   Fri, 9 Mar 2018 09:24:59 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Artem Bityutskiy <dedekind1@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        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

Hi Thomas,

On Fri, Mar 09, 2018 at 12:20:09AM +0100, Thomas Gleixner wrote:
> On Thu, 8 Mar 2018, Ming Lei wrote:
> > 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
> 
> Where are these commits? Neither Linus tree not -next know anything about
> them....

Both aren't merged yet, but they should land V4.16, IMO.

> 
> > 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.
> 
> So my understanding is that these irq patches are enhancements and not bug
> fixes. I'll queue them for 4.17 then.

Wrt. this IO hang issue, these patches shouldn't be bug fix, but they may
fix performance regression[1] for some systems caused by 84676c1f21 ("genirq/affinity:
assign vectors to all possible CPUs").

[1] https://marc.info/?l=linux-block&m=152050347831149&w=2

Thanks,
Ming