linux-kernel - Re: [PATCH] blk-mq: fix selecting software ctx for request

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Message-ID: <20200203122946.GC31450@ming.t460p>
Date:   Mon, 3 Feb 2020 20:29:46 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     linux-block@...r.kernel.org, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-mq: fix selecting software ctx for request

On Mon, Feb 03, 2020 at 06:28:15PM +0800, Hillf Danton wrote:
> 
> Hi Ming
> 
> On Mon, 3 Feb 2020 17:16:02 +0800 Ming Lei wrote:
> > On Sun, Feb 02, 2020 at 06:20:04PM +0800, Hillf Danton wrote:
> > > 
> > > Select the current cpu if it's mapped to hardware to make helpers like
> > > blk_mq_rq_cpu() return correct value.
> > > 
> > > Signed-off-by: Hillf Danton <hdanton@...a.com>
> > > ---
> > > 
> > > --- a/block/blk-mq.c
> > > +++ b/block/blk-mq.c
> > > @@ -454,7 +454,10 @@ struct request *blk_mq_alloc_request_hct
> > >  		blk_queue_exit(q);
> > >  		return ERR_PTR(-EXDEV);
> > >  	}
> > > -	cpu = cpumask_first_and(alloc_data.hctx->cpumask, cpu_online_mask);
> > > +	cpu = raw_smp_processor_id();
> > > +	if (!cpumask_test_cpu(cpu, alloc_data.hctx->cpumask))
> > > +		cpu = cpumask_first_and(alloc_data.hctx->cpumask,
> > > +						cpu_online_mask);
> > 
> > How can you know if there is any online CPU available for this hctx?
> 
> I don't except for the current cpu.

The current CPU may not belong to hctx->cpumask for blk_mq_alloc_request_hctx(),
that is why I think this API is very weird.


Thanks, 
Ming