Message-ID: <aFwI6sCtObAGoMBt@localhost.localdomain>
Date: Wed, 25 Jun 2025 16:34:18 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Phil Auld <pauld@...hat.com>
Cc: Waiman Long <llong@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Marco Crivellari <marco.crivellari@...e.com>,
	Michal Hocko <mhocko@...e.com>,
	Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH 02/27] sched/isolation: Introduce housekeeping per-cpu
 rwsem

Le Wed, Jun 25, 2025 at 08:18:50AM -0400, Phil Auld a écrit :
> Hi Waiman,
> 
> On Mon, Jun 23, 2025 at 01:34:58PM -0400 Waiman Long wrote:
> > On 6/20/25 11:22 AM, Frederic Weisbecker wrote:
> > > The HK_TYPE_DOMAIN isolation cpumask, and further the
> > > HK_TYPE_KERNEL_NOISE cpumask will be made modifiable at runtime in the
> > > future.
> > > 
> > > The affected subsystems will need to synchronize against those cpumask
> > > changes so that:
> > > 
> > > * The reader get a coherent snapshot
> > > * The housekeeping subsystem can safely propagate a cpumask update to
> > >    the susbsytems after it has been published.
> > > 
> > > Protect against readsides that can sleep with per-cpu rwsem. Updates are
> > > expected to be very rare given that CPU isolation is a niche usecase and
> > > related cpuset setup happen only in preparation work. On the other hand
> > > read sides can occur in more frequent paths.
> > > 
> > > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > 
> > Thanks for the patch series and it certainly has some good ideas. However I
> > am a bit concern about the overhead of using percpu-rwsem for
> > synchronization especially when the readers have to wait for the completion
> > on the writer side. From my point of view, during the transition period when
> > new isolated CPUs are being added or old ones being removed, the reader will
> > either get the old CPU data or the new one depending on the exact timing.
> > The effect the CPU selection may persist for a while after the end of the
> > critical section.
> > 
> > Can we just rely on RCU to make sure that it either get the new one or the
> > old one but nothing in between without the additional overhead?
> > 
> > My current thinking is to make use CPU hotplug to enable better CPU
> > isolation. IOW, I would shut down the affected CPUs, change the housekeeping
> > masks and then bring them back online again. That means the writer side will
> > take a while to complete.
> 
> The problem with this approach is that offlining a cpu effects all the other
> cpus and causes latency spikes on other low latency tasks which may already be
> running on other parts of the system.
> 
> I just don't want us to finally get to dynamic isolation and have it not
> usable for the usecases asking for it.

We'll have to discuss that eventually because that's the plan for nohz_full.
We can work around the stop machine rendez-vous on nohz_full if that's the
problem. If the issue is not to interrupt common RT-tasks, then that's a
different problem for which I don't have a solution.

Thanks.

> 
> Cheers,
> Phil
> 
> > 
> > Cheers,
> > Longman
> > 
> > > ---
> > >   include/linux/sched/isolation.h |  7 +++++++
> > >   kernel/sched/isolation.c        | 12 ++++++++++++
> > >   kernel/sched/sched.h            |  1 +
> > >   3 files changed, 20 insertions(+)
> > > 
> > > diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
> > > index f98ba0d71c52..8de4f625a5c1 100644
> > > --- a/include/linux/sched/isolation.h
> > > +++ b/include/linux/sched/isolation.h
> > > @@ -41,6 +41,9 @@ static inline bool housekeeping_cpu(int cpu, enum hk_type type)
> > >   		return true;
> > >   }
> > > +extern void housekeeping_lock(void);
> > > +extern void housekeeping_unlock(void);
> > > +
> > >   extern void __init housekeeping_init(void);
> > >   #else
> > > @@ -73,6 +76,8 @@ static inline bool housekeeping_cpu(int cpu, enum hk_type type)
> > >   	return true;
> > >   }
> > > +static inline void housekeeping_lock(void) { }
> > > +static inline void housekeeping_unlock(void) { }
> > >   static inline void housekeeping_init(void) { }
> > >   #endif /* CONFIG_CPU_ISOLATION */
> > > @@ -84,4 +89,6 @@ static inline bool cpu_is_isolated(int cpu)
> > >   	       cpuset_cpu_is_isolated(cpu);
> > >   }
> > > +DEFINE_LOCK_GUARD_0(housekeeping, housekeeping_lock(), housekeeping_unlock())
> > > +
> > >   #endif /* _LINUX_SCHED_ISOLATION_H */
> > > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> > > index 83cec3853864..8c02eeccea3b 100644
> > > --- a/kernel/sched/isolation.c
> > > +++ b/kernel/sched/isolation.c
> > > @@ -18,12 +18,24 @@ static cpumask_var_t housekeeping_cpumasks[HK_TYPE_MAX];
> > >   unsigned long housekeeping_flags;
> > >   EXPORT_SYMBOL_GPL(housekeeping_flags);
> > > +DEFINE_STATIC_PERCPU_RWSEM(housekeeping_pcpu_lock);
> > > +
> > >   bool housekeeping_enabled(enum hk_type type)
> > >   {
> > >   	return !!(housekeeping_flags & BIT(type));
> > >   }
> > >   EXPORT_SYMBOL_GPL(housekeeping_enabled);
> > > +void housekeeping_lock(void)
> > > +{
> > > +	percpu_down_read(&housekeeping_pcpu_lock);
> > > +}
> > > +
> > > +void housekeeping_unlock(void)
> > > +{
> > > +	percpu_up_read(&housekeeping_pcpu_lock);
> > > +}
> > > +
> > >   int housekeeping_any_cpu(enum hk_type type)
> > >   {
> > >   	int cpu;
> > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > > index 475bb5998295..0cdb560ef2f3 100644
> > > --- a/kernel/sched/sched.h
> > > +++ b/kernel/sched/sched.h
> > > @@ -46,6 +46,7 @@
> > >   #include <linux/mm.h>
> > >   #include <linux/module.h>
> > >   #include <linux/mutex_api.h>
> > > +#include <linux/percpu-rwsem.h>
> > >   #include <linux/plist.h>
> > >   #include <linux/poll.h>
> > >   #include <linux/proc_fs.h>
> > 
> > 
> 
> -- 
> 

-- 
Frederic Weisbecker
SUSE Labs

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