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

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Message-ID: <CAHttsra_jACOSZpwnp0KtuAOFWtXt5AHf+RVWMvEbxWbieVw0w@mail.gmail.com>
Date:   Tue, 7 May 2019 10:21:58 +0800
From:   Yuyang Du <duyuyang@...il.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, will.deacon@....com,
        Ingo Molnar <mingo@...nel.org>,
        Bart Van Assche <bvanassche@....org>, ming.lei@...hat.com,
        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 Tue, 7 May 2019 at 09:47, Frederic Weisbecker <frederic@...nel.org> wrote:
> > > 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.
>
> Right, so in my patchset there is indeed individual soft vectors masked
> so we indeed need per vector checks. For example a lock taken in HRTIMER
> softirq shouldn't be a problem if it is concurrently taken while BLOCK softirq
> is enabled. And for that we expand the usage_mask so that the 4 bits currently
> used for general SOFTIRQ are now multiplied by NR_SOFTIRQ (10) because we need to
> track the USED and ENABLED_IN bits for each of them.
>
> The end result is:
>
> 4 hard irq bits + 4 * 10 softirq bits + LOCK_USED bit = 45 bits.
>
> Not sure that answers the question as I'm a bit lost in the debate...

It was really I was lost: I didn't realize the enabling (or disabling)
is going to be fine-grained as well until I read this changelog:

Disabling the softirqs is currently an all-or-nothing operation: either
all softirqs are enabled or none of them. However we plan to introduce a
per vector granularity of this ability to improve latency response and
make each softirq vector interruptible by the others.

Sorry for the confusion I made :)