linux-kernel - Re: [PATCH 19/28] locking/lockdep: Optimize irq usage check when marking lock usage bit

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Date:   Mon, 6 May 2019 11:42:05 +0800
From:   Yuyang Du <duyuyang@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     will.deacon@....com, Ingo Molnar <mingo@...nel.org>,
        Bart Van Assche <bvanassche@....org>, ming.lei@...hat.com,
        Frederic Weisbecker <frederic@...nel.org>, tglx@...utronix.de,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 19/28] locking/lockdep: Optimize irq usage check when
 marking lock usage bit

On Mon, 6 May 2019 at 11:05, Yuyang Du <duyuyang@...il.com> wrote:
>
> On Tue, 30 Apr 2019 at 20:12, Peter Zijlstra <peterz@...radead.org> wrote:
> > > > IOW he's going to massively explode this storage.
> > >
> > > If I understand correctly, he is not going to.
> > >
> > > First of all, we can divide the whole usage thing into tracking and checking.
> > >
> > > Frederic's fine-grained soft vector state is applied to usage
> > > tracking, i.e., which specific vectors a lock is used or enabled.
> > >
> > > But for usage checking, which vectors are does not really matter. So,
> > > the current size of the arrays and bitmaps are good enough. Right?
> >
> > Frederic? My understanding was that he really was going to split the
> > whole thing. The moment you allow masking individual soft vectors, you
> > get per-vector dependency chains.
>
> It seems so. What I understand is: for IRQ usage, the difference is:
>
> Each lock has a new usage mask:
>
>         softirq10, ..., softirq1, hardirq
>
> where softirq1 | softirq2 | ... | softirq10 = softirq
>
> where softirq, exactly what was, virtually is used in the checking.
> This is mainly because, any irq vector has any usage, the lock has
> that usage, be it hard or soft.
>
> If that is right, hardirq can be split too (why not if softirq does
> :)). So, maybe a bitmap to do them all for tracking, and optionally
> maintain aggregate softirq and hardirq for checking as before.
> Regardless, may irq-safe reachability thing is not affected.
>
> And for the chain, which is mainly for caching does not really matter
> split or not (either way, the outcome will be the same?), because
> there will be a hash for a chain anyway, which is the same. Right?

Oh, another look at the patch, I was wrong, it can be very different
if consider: used in vector X vs. enabled on vector Y (which is ok),
when enablement can be so fine-grained as well, which is actually the
point of the patch, though no difference for now :(