Re: [RFC PATCH 0/8] Introduce simple hazard pointers for lockdep

Uladzislau Rezki
2025-04-17T08:22:26Z

Hello, Breno!

> Hello Vlad,
>
> On Wed, Apr 16, 2025 at 05:04:31PM +0200, Uladzislau Rezki wrote:
> > On Wed, Apr 16, 2025 at 07:14:04AM -0700, Breno Leitao wrote:
> > > Hi Boqun,
> > >
> > > On Sun, Apr 13, 2025 at 11:00:47PM -0700, Boqun Feng wrote:
> > >
> > > > Overall it looks promising to me, but I would like to see how it
> > > > performs in the environment of Breno. Also as Paul always reminds me:
> > > > buggy code usually run faster, so please take a look in case I'm missing
> > > > something ;-) Thanks!
> > >
> > > Thanks for the patchset. I've confirmed that the wins are large on my
> > > environment, but, at the same magnitute of synchronize_rcu_expedited().
> > >
> > > Here are the numbers I got:
> > >
> > > 6.15-rc1 (upstream)
> > > # time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1234: mq
> > > real 0m3.986s
> > > user 0m0.001s
> > > sys 0m0.093s
> > >
> > > Your patchset on top of 6.15-rc1
> > > # time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1234: mq
> > > real 0m0.072s
> > > user 0m0.001s
> > > sys 0m0.070s
> > >
> > >
> > > My original proposal of using synchronize_rcu_expedited()[1]
> > > # time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1234: mq
> > > real 0m0.074s
> > > user 0m0.001s
> > > sys 0m0.061s
> > >
> > > Link: https://lore.kernel.org/all/20250321-lockdep-v1-1-78b732d195fb@debian.org/ [1]
> > >
> > Could you please also do the test of fist scenario with a regular
> > synchronize_rcu() but switch to its faster variant:
> >
> > echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
> >
> > and run the test. If you have a time.
>
> Of course, I am more than interesting in this topic. This is what I run:
>
>
> # /usr/sbin/tc qdisc replace dev eth0 root handle 0x1: mq; time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1234: mq
> real 0m4.150s
> user 0m0.001s
> sys 0m0.076s
>
> [root@host2 ~]# echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
> [root@host2 ~]# /usr/sbin/tc qdisc replace dev eth0 root handle 0x1: mq; time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1234: mq
> real 0m4.225s
> user 0m0.000s
> sys 0m0.106s
>
> [root@host2 ~]# cat /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
> 1
> [root@host2 ~]# echo 0 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
> [root@host2 ~]# /usr/sbin/tc qdisc replace dev eth0 root handle 0x1: mq; time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1234: mq
> real 0m4.152s
> user 0m0.001s
> sys 0m0.099s
>
> It seems it made very little difference?
>
Yep, no difference. In your case you just need to speed up a grace
period completion. So an expedited version really improves your case.

So you do not have a lot of callbacks which may delay a normal GP.

Thank you!

--
Uladzislau Rezki