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

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Date:   Thu, 17 Nov 2022 14:11:38 +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 Thu, Nov 17, 2022 at 08:06:21AM -0500, Joel Fernandes wrote:
> 
> 
> > On Nov 17, 2022, at 7:58 AM, Uladzislau Rezki <urezki@...il.com> wrote:
> > 
> > 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.
> > 
> Ah, ok.
> 
> >> 
> >> Also it still does cond_synchronize_rcu() :-(
> >> 
> > Sometimes we need to wait for a GP we can not just release :)
> 
> You know that is not what I meant ;) I was concerned about the blocking.
> 
Let me split. After that we/you can test and check if there is any issue
with sleeping on entry for waiting a GP if needed.

--
Uladzislau Rezki