linux-kernel - Re: [PATCH 2/5] kernel/watchdog_hld: clarify the condition in hardlockup_detector_event

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Message-ID: <YULAuEeSSeLTcBM9@piliu.users.ipa.redhat.com>
Date:   Thu, 16 Sep 2021 11:57:44 +0800
From:   Pingfan Liu <piliu@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Pingfan Liu <kernelfans@...il.com>, linux-kernel@...r.kernel.org,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wang Qing <wangqing@...o.com>,
        Santosh Sivaraj <santosh@...six.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 2/5] kernel/watchdog_hld: clarify the condition in
 hardlockup_detector_event_create()

On Wed, Sep 15, 2021 at 03:45:06PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 15, 2021 at 11:51:00AM +0800, Pingfan Liu wrote:
> > hardlockup_detector_event_create() indirectly calls
> > kmem_cache_alloc_node(), which is blockable.
> > 
> > So here, the really planned context is is_percpu_thread().
> > 
> > Signed-off-by: Pingfan Liu <kernelfans@...il.com>
> > Cc: Petr Mladek <pmladek@...e.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Wang Qing <wangqing@...o.com>
> > Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> > Cc: Santosh Sivaraj <santosh@...six.org>
> > Cc: Sumit Garg <sumit.garg@...aro.org>
> > Cc: Will Deacon <will@...nel.org>
> > Cc: Mark Rutland <mark.rutland@....com>
> > To: linux-kernel@...r.kernel.org
> > ---
> >  kernel/watchdog_hld.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
> > index 247bf0b1582c..6876e796dbf5 100644
> > --- a/kernel/watchdog_hld.c
> > +++ b/kernel/watchdog_hld.c
> > @@ -165,10 +165,13 @@ static void watchdog_overflow_callback(struct perf_event *event,
> >  
> >  static int hardlockup_detector_event_create(void)
> >  {
> > -	unsigned int cpu = smp_processor_id();
> > +	unsigned int cpu;
> >  	struct perf_event_attr *wd_attr;
> >  	struct perf_event *evt;
> >  
> > +	/* This function plans to execute in cpu bound kthread */
> > +	BUG_ON(!is_percpu_thread());
> > +	cpu = raw_smp_processor_id();
> >  	wd_attr = &wd_hw_attr;
> >  	wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);
> 
> This patch makes no sense.

This patch aims to disable any attempt such as using get_cpu()/put_cpu() to
shut up the check_preemption_disabled().

But if anybody is familiar with the integration of watchdog_hld and
cpuhp, he should know the right way without this BUG_ON() or warn.

Do you still think it is pointless?


Thanks,

	Pingfan