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

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Date:   Thu, 17 Nov 2022 08:06:21 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     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 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.

Thanks,

 - Joel 

> 
> --
> Uladzislau Rezki