linux-kernel - Re: [patch 1/2] nohz: try to avoid IPI when configuring per-CPU posix timer

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Message-ID: <20200902194240.GA986833@fuller.cnet>
Date:   Wed, 2 Sep 2020 16:42:40 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [patch 1/2] nohz: try to avoid IPI when configuring per-CPU
 posix timer

On Wed, Sep 02, 2020 at 01:38:59AM +0200, Frederic Weisbecker wrote:
> On Tue, Aug 25, 2020 at 03:41:48PM -0300, Marcelo Tosatti wrote:
> > When enabling per-CPU posix timers, an IPI to nohz_full CPUs might be
> > performed (to re-read the dependencies and possibly not re-enter
> > nohz_full on a given CPU).
> > 
> > A common case is for applications that run on nohz_full= CPUs 
> > to not use POSIX timers (eg DPDK). This patch skips the IPI 
> > in case the task allowed mask does not intersect with nohz_full= CPU mask,
> > when going through tick_nohz_dep_set_signal.
> > 
> > This reduces interruptions to nohz_full= CPUs.
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> [...]
> >  /*
> > + * Set bit on nohz full dependency, kicking all cpus
> > + * only if task can run on nohz full CPUs.
> > + */
> > +static void tick_nohz_dep_set_all_cond(struct task_struct *tsk,
> > +				       atomic_t *dep,
> > +				       enum tick_dep_bits bit)
> > +{
> > +	int prev;
> > +	unsigned long flags;
> > +
> > +	prev = atomic_fetch_or(BIT(bit), dep);
> > +	if (prev)
> > +		return;
> > +
> > +	raw_spin_lock_irqsave(&tsk->pi_lock, flags);
> > +	if (cpumask_intersects(&tsk->cpus_mask, tick_nohz_full_mask))
> > +		tick_nohz_full_kick_all();
> 
> So that's for one task but what about the other threads in that
> process? We are setting the tick dependency on all tasks sharing that
> struct signal.

Hi Frederic,

Yep, fixing in -v2, thanks.