linux-kernel - Re: [PATCH v2] rcu/kfree: Do not request RCU when not needed

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Message-ID: <Y3YwDuKibmOiz6/7@pc636>
Date:   Thu, 17 Nov 2022 13:58:54 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Uladzislau Rezki <urezki@...il.com>, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, rcu@...r.kernel.org
Subject: Re: [PATCH v2] rcu/kfree: Do not request RCU when not needed

On Wed, Nov 16, 2022 at 10:05:46PM +0000, Joel Fernandes wrote:
> On Wed, Nov 16, 2022 at 7:19 PM Uladzislau Rezki <urezki@...il.com> wrote:
> >
> > Hello, Paul, Joel.
> >
> > > >
> > > > Yes sure, I am doing a run now with my patch. However, I have a
> > > > question -- why do you feel blocking in the kworker is not an issue?
> > > > You are taking a snapshot before queuing the normal kwork and then
> > > > reading the snapshot when the normal kwork runs. Considering it is a
> > > > high priority queue, the delay between when you are taking the
> > > > snapshot, and reading it is likely small so there is a bigger chance
> > > > of blocking in cond_synchronize_rcu(). Did I miss something?
> > > >
> > > We can wait indeed in the reclaim worker. But the worker does not do any
> > > nasty or extra work here. If there is a need we block and wait. After a
> > > grace period, we are awoken and proceed.
> > >
> > > Therefore i do not see the reason in handling two cases:
> > >
> > > if (gp_done)
> > >     queue_work();
> > > else
> > >     queue_rcu_work();
> > >
> > > it is the same if we just queue the work and check on entry. The current
> > > scenario is: queue the work after a grace period. This is the difference.
> > >
> > > Right if the reclaimer was a high prio kthread a time would be shorter.
> > >
> > > In your scenario the time seems even shorter(i have not checked) because
> > > you update a snapshot of krcp each time a kvfree_rcu() is invoked. So
> > > basically even though you have objects whose grace period is passed you
> > > do not separate it anyhow. Because you update the:
> > >
> > > krcp->gp_snap = get_state_synchronize_rcu();
> > >
> > > too often.
> > >
> > Once upon a time we discussed that it is worth to keep track of GP
> > per-a-page in order to reduce a memory footprint. Below patch addresses
> > it:
> 
> In the patch below, it appears you are tracking the GP per krwp, and
> not per page. But I could be missing something - could you split it
> into separate patches for easier review?
>
I will split. I was thinking about it. The GP is tracked per-a-page. As for
krwp it is only for channel_3. Everything goes there if no-page or no cache.

> 
> Also it still does cond_synchronize_rcu() :-(
> 
Sometimes we need to wait for a GP we can not just release :)

--
Uladzislau Rezki