Re: [PATCH V2] lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus_evenly

[Ming Lei <ming.lei@redhat.com>]

On Fri, Aug 18, 2023 at 02:59:13PM +0800, Chengming Zhou wrote:
> Hi,
> 
> On 2023/8/18 09:52, Ming Lei wrote:
> > group_cpus_evenly() could be part of storage driver's error handler,
> > such as nvme driver, when may happen during CPU hotplug, in which
> > storage queue has to drain its pending IOs because all CPUs associated
> > with the queue are offline and the queue is becoming inactive. And
> > handling IO needs error handler to provide forward progress.
> > 
> > Then dead lock is caused:
> > 
> > 1) inside CPU hotplug handler, CPU hotplug lock is held, and blk-mq's
> > handler is waiting for inflight IO
> > 
> > 2) error handler is waiting for CPU hotplug lock
> > 
> > 3) inflight IO can't be completed in blk-mq's CPU hotplug handler because
> > error handling can't provide forward progress.
> > 
> > Solve the deadlock by not holding CPU hotplug lock in group_cpus_evenly(),
> > in which two stage spreads are taken: 1) the 1st stage is over all present
> > CPUs; 2) the end stage is over all other CPUs.
> > 
> > Turns out the two stage spread just needs consistent 'cpu_present_mask', and
> > remove the CPU hotplug lock by storing it into one local cache. This way
> > doesn't change correctness, because all CPUs are still covered.
> > 
> > Cc: Keith Busch <kbusch@kernel.org>
> > Cc: linux-nvme@lists.infradead.org
> > Cc: linux-block@vger.kernel.org
> > Reported-by: Yi Zhang <yi.zhang@redhat.com>
> > Reported-by: Guangwu Zhang <guazhang@redhat.com>
> > Tested-by: Guangwu Zhang <guazhang@redhat.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> > V2:
> > 	- fix "Cc: block list"
> > 	- add tested-by tag
> > 
> >  lib/group_cpus.c | 22 ++++++++++++++++------
> >  1 file changed, 16 insertions(+), 6 deletions(-)
> > 
> > diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> > index aa3f6815bb12..15006e79196f 100644
> > --- a/lib/group_cpus.c
> > +++ b/lib/group_cpus.c
> > @@ -348,6 +348,7 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
> >  {
> >  	unsigned int curgrp = 0, nr_present = 0, nr_others = 0;
> >  	cpumask_var_t *node_to_cpumask;
> > +	cpumask_var_t local_cpu_present_mask;
> >  	cpumask_var_t nmsk, npresmsk;
> >  	int ret = -ENOMEM;
> >  	struct cpumask *masks = NULL;
> > @@ -355,6 +356,16 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
> >  	if (!zalloc_cpumask_var(&nmsk, GFP_KERNEL))
> >  		return NULL;
> >  
> > +	if (!zalloc_cpumask_var(&local_cpu_present_mask, GFP_KERNEL))
> > +		goto fail_local_pres_mask;
> > +
> > +	/*
> > +	 * Make a local cache of 'cpu_present_mask', so the two stages
> > +	 * spread can observe consistent 'cpu_present_mask' without holding
> > +	 * cpu hotplug lock.
> > +	 */
> > +	cpumask_copy(local_cpu_present_mask, cpu_present_mask);
> > +
> 
> Maybe we can reuse npresmsk instead of allocating another cpumask?
> In the first stage: npresmsk = cpu_present_mask
> In the second stage: npresmsk = cpu_possible_mask & ~npresmsk

Good idea!


Thanks, 
Ming