lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Fri, 1 Oct 2021 17:13:31 -0700
From:   Roman Gushchin <>
To:     Barry Song <>
CC:     Peter Zijlstra <>,
        Ingo Molnar <>,
        Mel Gorman <>,
        <>, LKML <>
Subject: Re: [PATCH rfc 4/6] sched: cfs: add bpf hooks to control wakeup and
 tick preemption

On Fri, Oct 01, 2021 at 04:35:58PM +1300, Barry Song wrote:
> On Fri, Sep 17, 2021 at 4:36 AM Roman Gushchin <> wrote:
> >
> > This patch adds 3 hooks to control wakeup and tick preemption:
> >   cfs_check_preempt_tick
> >   cfs_check_preempt_wakeup
> >   cfs_wakeup_preempt_entity
> >
> > The first one allows to force or suppress a preemption from a tick
> > context. An obvious usage example is to minimize the number of
> > non-voluntary context switches and decrease an associated latency
> > penalty by (conditionally) providing tasks or task groups an extended
> > execution slice. It can be used instead of tweaking
> > sysctl_sched_min_granularity.
> >
> > The second one is called from the wakeup preemption code and allows
> > to redefine whether a newly woken task should preempt the execution
> > of the current task. This is useful to minimize a number of
> > preemptions of latency sensitive tasks. To some extent it's a more
> > flexible analog of a sysctl_sched_wakeup_granularity.
> This reminds me of Mel's recent work which might be relevant:
> sched/fair: Scale wakeup granularity relative to nr_running

Oh, this is interesting, thank you for the link! This is a perfect example
of a case when bpf can be useful if the change will be considered to be too
special to be accepted in the mainline code.

> >
> > The third one is similar, but it tweaks the wakeup_preempt_entity()
> > function, which is called not only from a wakeup context, but also
> > from pick_next_task(), which allows to influence the decision on which
> > task will be running next.
> >
> > It's a place for a discussion whether we need both these hooks or only
> > one of them: the second is more powerful, but depends more on the
> > current implementation. In any case, bpf hooks are not an ABI, so it's
> > not a deal breaker.
> I am also curious if similar hook can benefit
> newidle_balance/sched_migration_cost
> tuning things in this thread:
> It seems those static values are not universal. different topology might need
> different settings.  but dynamically tuning them in the kernel seems to be
> extremely difficult.

Absolutely! I'm already playing with newidle_balance (no specific results yet).
And sched_migration_cost is likely a good target too!


Powered by blists - more mailing lists