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Date:   Wed, 14 Aug 2019 16:58:46 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     "Derrick, Jonathan" <jonathan.derrick@...el.com>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "kbusch@...nel.org" <kbusch@...nel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>, "hch@....de" <hch@....de>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3 1/3] genirq/affinity: Enhance warning check

On Tue, Aug 13, 2019 at 07:31:39PM +0000, Derrick, Jonathan wrote:
> Hi Ming,
> 
> On Tue, 2019-08-13 at 16:14 +0800, Ming Lei wrote:
> > The two-stage spread is done on same irq vectors, and we just need that
> > either one stage covers all vector, not two stage work together to cover
> > all vectors.
> > 
> > So enhance the warning check to make sure all vectors are spread.
> > 
> > Cc: Christoph Hellwig <hch@....de>
> > Cc: Keith Busch <kbusch@...nel.org>
> > Cc: linux-nvme@...ts.infradead.org,
> > Cc: Jon Derrick <jonathan.derrick@...el.com>
> > Cc: Jens Axboe <axboe@...nel.dk>
> > Fixes: 6da4b3ab9a6 ("genirq/affinity: Add support for allocating interrupt sets")
> > Signed-off-by: Ming Lei <ming.lei@...hat.com>
> > ---
> >  kernel/irq/affinity.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
> > index 6fef48033f96..265b3076f16b 100644
> > --- a/kernel/irq/affinity.c
> > +++ b/kernel/irq/affinity.c
> > @@ -215,8 +215,7 @@ static int irq_build_affinity_masks(unsigned int startvec, unsigned int numvecs,
> >  					       npresmsk, nmsk, masks);
> >  	put_online_cpus();
> >  
> > -	if (nr_present < numvecs)
> > -		WARN_ON(nr_present + nr_others < numvecs);
> > +	WARN_ON(max(nr_present, nr_others) < numvecs);
> 
> I think the patch description assumes the first condition
> "The two-stage spread is done on same irq vectors"
> 
>     /*
>      * Spread on non present CPUs starting from the next vector to be
>      * handled. If the spreading of present CPUs already exhausted the
>      * vector space, assign the non present CPUs to the already spread
>      * out vectors.
>      */
>     if (nr_present >= numvecs)
>             curvec = firstvec;
> 
> But doesn't following condition imply nr_others spread is potentionally
> different vector set?
> 
>     else
>             curvec = firstvec + nr_present;
> 

Most times, __irq_build_affinity_masks() returns numvecs.

However, each stage may expose less CPU number than num_vecs, then less
vectors than 'numvecs' can be spread.

So this patch is actually wrong.


Thank,
Ming

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