linux-kernel - Re: [PATCH] sched/wait: introduce wait_event_freezable

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Date:   Fri, 18 Jan 2019 18:08:01 +0100
From:   Hugo Lefeuvre <hle@....eu.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Greg Hartman <ghartman@...gle.com>,
        Alistair Strachan <strachan@...gle.com>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Christian Brauner <christian@...uner.io>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/wait: introduce wait_event_freezable_hrtimeout

Hi Joel,

Thanks for your review.

> I believe these should be 2 patches. In the first patch you introduce the
> new API, in the second one you would simplify the VSOC driver.
> 
> In fact, in one part of the patch you are using wait_event_freezable which
> already exists so that's unrelated to the new API you are adding.

Agree, I will split the patch for the v2.

> > +/*
> > + * like wait_event_hrtimeout() -- except it uses TASK_INTERRUPTIBLE to avoid
> > + * increasing load and is freezable.
> > + */
> > +#define wait_event_freezable_hrtimeout(wq_head, condition, timeout)		\
> 
> You should document the variable names in the header comments.

Agree. This comment was copy/pasted from wait_event_freezable_timeout,
should I fix it there as well?

> Also, this new API appears to conflict with definition of 'freezable' in
> wait_event_freezable I think,
> 
> wait_event_freezable - sleep or freeze until condition is true.
> 
> wait_event_freezable_hrtimeout - sleep but make sure freezer is not blocked.
> (your API)
> 
> It seems to me these are 2 different definitions of 'freezing' (or I could be
> completely confused). But in the first case it calls try_to_freeze after
> schedule(). In the second case (your new API), it calls freezable_schedule().
> 
> So I am wondering why is there this difference in the 'meaning of freezable'.
> In the very least, any such subtle differences should be documented in the
> header comments IMO.

It appears that freezable_schedule() and schedule(); try_to_freeze() are
almost identical:

    static inline void freezable_schedule(void)
    {
        freezer_do_not_count();
        schedule();
        freezer_count();
    }

and

    static inline void freezer_do_not_count(void)
    {
        current->flags |= PF_FREEZER_SKIP;
    }

    static inline void freezer_count(void)
    {
        current->flags &= ~PF_FREEZER_SKIP;
        /*
         * If freezing is in progress, the following paired with smp_mb()
         * in freezer_should_skip() ensures that either we see %true
         * freezing() or freezer_should_skip() sees !PF_FREEZER_SKIP.
         */
        smp_mb();
        try_to_freeze();
    }

as far as I understand this code, freezable_schedule() avoids blocking the
freezer during the schedule() call, but in the end try_to_freeze() is still
called so the result is the same, right?

I wonder why wait_event_freezable is not calling freezable_schedule().

That being said, I am not sure that the try_to_freeze() call does anything
in the vsoc case because there is no call to set_freezable() so the thread
still has PF_NOFREEZE...

regards,
 Hugo

-- 
                Hugo Lefeuvre (hle)    |    www.owl.eu.com
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