linux-kernel - Re: [RFC PATCH 11/14] context-tracking: Introduce work deferral infrastructure

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Message-ID: <xhsmhlefsvrtu.mognet@vschneid.remote.csb>
Date:   Thu, 06 Jul 2023 19:05:17 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     paulmck@...nel.org, Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org,
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        Yair Podemsky <ypodemsk@...hat.com>
Subject: Re: [RFC PATCH 11/14] context-tracking: Introduce work deferral
 infrastructure

On 06/07/23 09:39, Paul E. McKenney wrote:
> On Thu, Jul 06, 2023 at 01:40:14PM +0200, Frederic Weisbecker wrote:
>> On Thu, Jul 06, 2023 at 12:30:46PM +0100, Valentin Schneider wrote:
>> > I'm trying to grok how this impacts RCU, IIUC most of RCU mostly cares about the
>> > even/odd-ness of the thing, and rcu_gp_fqs() cares about the actual value
>> > but only to check if it has changed over time (rcu_dynticks_in_eqs_since()
>> > only does a !=).
>> >
>> > I'm rephrasing here to make sure I get it - is it then that the worst case
>> > here is 2^(dynticks_counter_size) transitions happen between saving the
>> > dynticks snapshot and checking it again, so RCU waits some more?
>>
>> That's my understanding as well but I have to defer on Paul to make sure I'm
>> not overlooking something.
>
> That does look plausible to me.
>
> And yes, RCU really cares about whether its part of this counter has
> been a multiple of two during a given interval of time, because this
> indicates that the CPU has no pre-existing RCU readers still active.
> One way that this can happen is for that value to be a multiple of two
> at some point in time.  The other way that this can happen is for the
> value to have changed.  No matter what the start and end values, if they
> are different, the counter must necessarily have at least passed through
> multiple of two in the meantime, again guaranteeing that any RCU readers
> that around when the count was first fetched have now finished.
>

Thank you for the demystification!

> But we should take the machine's opinions much more seriously than we
> take any of our own opinions.

Heh :-)

> Why not adjust RCU_DYNTICKS_IDX so as
> to crank RCU's portion of this counter down to (say) two or three bits
> and let rcutorture have at it on TREE04 or TREE07, both of which have
> nohz_full CPUs?
>
> Maybe also adjust mkinitrd.sh to make the user/kernel transitions more
> frequent?
>
> Please note that I do -not- recommend production use of a three-bit
> (let alone a two-bit) RCU portion because this has a high probability
> of excessively extending grace periods.  But it might be good to keep
> a tiny counter as a debug option so that we regularly rcutorture it.
>

Sounds sensible, I'll add that to my v2 todolist.

Thanks!

>                                                       Thanx, Paul