linux-kernel - Re: [PATCH rcu/dev -fixes 2/4] rcu/tree: Skip entry into the page allocator for PREEMPT

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Date:   Wed, 22 Apr 2020 09:18:41 -0400
From:   joel@...lfernandes.org
To:     Uladzislau Rezki <urezki@...il.com>
CC:     linux-kernel@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH rcu/dev -fixes 2/4] rcu/tree: Skip entry into the page allocator for PREEMPT_RT



On April 22, 2020 6:35:36 AM EDT, Uladzislau Rezki <urezki@...il.com> wrote:
>On Mon, Apr 20, 2020 at 11:38:35AM -0400, Joel Fernandes (Google)
>wrote:
>> To keep kfree_rcu() path working on raw non-preemptible sections,
>> prevent the optional entry into the allocator as it uses sleeping
>locks.
>> In fact, even if the caller of kfree_rcu() is preemptible, this path
>> still is not, as krcp->lock is a raw spinlock as done in previous
>> patches. With additional page pre-allocation in the works, hitting
>this
>> return is going to be much less likely soon so just prevent it for
>now
>> so that PREEMPT_RT does not break. Note that page allocation here is
>an
>> optimization and skipping it still makes kfree_rcu() work.
>> 
>> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
>> Co-developed-by: Uladzislau Rezki <urezki@...il.com>
>> Signed-off-by: Uladzislau Rezki <urezki@...il.com>
>> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>> ---
>>  kernel/rcu/tree.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>> 
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index cf68d3d9f5b81..cd61649e1b001 100644
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -3092,6 +3092,18 @@ kfree_call_rcu_add_ptr_to_bulk(struct
>kfree_rcu_cpu *krcp,
>>  		if (!bnode) {
>>  			WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);
>>  
>> +			/*
>> +			 * To keep this path working on raw non-preemptible
>> +			 * sections, prevent the optional entry into the
>> +			 * allocator as it uses sleeping locks. In fact, even
>> +			 * if the caller of kfree_rcu() is preemptible, this
>> +			 * path still is not, as krcp->lock is a raw spinlock.
>> +			 * With additional page pre-allocation in the works,
>> +			 * hitting this return is going to be much less likely.
>> +			 */
>> +			if (IS_ENABLED(CONFIG_PREEMPT_RT))
>> +				return false;
>> +
>>  			bnode = (struct kfree_rcu_bulk_data *)
>>  				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
>>  		}
>This will not be correctly working by just reverting everything to the
>"rcu_head path" for CONFIG_PREEMPT_RT kernel. We need to preallocate at
>least once. I can add caching on top of this series to address it.
>

I discussed with Vlad offline, this patch is fine for mainline as we don't have headless support yet. So this patch is good. Future patches adding caching will also add the headless support after additional caching, so skipping the allocation here is ok.

Thanks.

- Joel




>--
>Vlad Rezki

-- 
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