linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] locking/osq: Use more optimized spinning for arm64
Date: Fri, 10 Jan 2020 09:13:26 -0500	[thread overview]
Message-ID: <c85eff71-2fcd-48e2-851f-96b4cd5a3f24@redhat.com> (raw)
In-Reply-To: <20200110100612.GC2827@hirez.programming.kicks-ass.net>

On 1/10/20 5:06 AM, Peter Zijlstra wrote:
> On Thu, Jan 09, 2020 at 10:38:31AM -0500, Waiman Long wrote:
>
>> --- a/kernel/locking/osq_lock.c
>> +++ b/kernel/locking/osq_lock.c
>> @@ -134,6 +134,27 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>>  	 * cmpxchg in an attempt to undo our queueing.
>>  	 */
>>  
>> +	/*
>> +	 * If vcpu_is_preempted is not defined, we can skip the check
>> +	 * and use smp_cond_load_relaxed() instead. For arm64, this
>> +	 * could lead to the use of the more optimized wfe instruction.
>> +	 * As need_sched() is set by interrupt handler, it will break
>> +	 * out and do the unqueue in a timely manner.
>> +	 *
>> +	 * TODO: We may need to add a static_key like vcpu_is_preemptible
>> +	 *	 as vcpu_is_preempted() will always return false with
>> +	 *	 bare metal even if it is defined.
>> +	 */
>> +#ifndef vcpu_is_preempted
>> +	{
>> +		int locked = smp_cond_load_relaxed(&node->locked,
>> +						   VAL || need_resched());
>> +		if (!locked)
>> +			goto unqueue;
>> +		return true;
>> +	}
>> +#endif
> Much yuck :-/
>
> With ARM64 being the only arch that currently makes use of this; another
> approach is doing something like:
>
> That is also rather yuck, and definitely needs a few comments sprinked
> on it, but it should just work for everyone.
>
> It basically relies on an arch having a spinning *cond_load*()
> implementation if it has vcpu_is_preempted(), which is true today.
>
> ---
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 6ef600aa0f47..6e00d7c077ba 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -133,18 +133,10 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>  	 * guaranteed their existence -- this allows us to apply
>  	 * cmpxchg in an attempt to undo our queueing.
>  	 */
> +	if (!smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
> +						  vcpu_is_preempetd(node_cpu(node->prev))))
> +		goto unqueue;
>  
> -	while (!READ_ONCE(node->locked)) {
> -		/*
> -		 * If we need to reschedule bail... so we can block.
> -		 * Use vcpu_is_preempted() to avoid waiting for a preempted
> -		 * lock holder:
> -		 */
> -		if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
> -			goto unqueue;
> -
> -		cpu_relax();
> -	}
>  	return true;
>  
>  unqueue:
>
Yes, that will work for now. We do need to document that in where
smp_cond_load_relaxed() is defined.

In the future, if vcpu_is_preempted() is defined for ARM64, it will
break. How about defining a variant like smp_cond_load_vcpu_relaxed(p,
cond, vcpu)? With that, we can make sure that the code will be properly
updated when vcpu_is_preempted() is defined for ARM64. I know it is
still kind of ugly, but it is a safer approach.

Cheers,
Longman


      reply	other threads:[~2020-01-10 14:13 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-09 15:38 Waiman Long
2020-01-10 10:06 ` Peter Zijlstra
2020-01-10 14:13   ` Waiman Long [this message]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=c85eff71-2fcd-48e2-851f-96b4cd5a3f24@redhat.com \
    --to=longman@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=peterz@infradead.org \
    --cc=will.deacon@arm.com \
    --subject='Re: [PATCH] locking/osq: Use more optimized spinning for arm64' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).