linux-kernel - Re: [RFC PATCH 36/86] entry: irqentry_exit only preempts TIF_NEED

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Message-ID: <87h6lfzlvz.fsf@oracle.com>
Date:   Mon, 20 Nov 2023 22:00:16 -0800
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, paulmck@...nel.org,
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        David.Laight@...lab.com, richard@....at, mjguzik@...il.com
Subject: Re: [RFC PATCH 36/86] entry: irqentry_exit only preempts
 TIF_NEED_RESCHED


Peter Zijlstra <peterz@...radead.org> writes:

> On Tue, Nov 07, 2023 at 01:57:22PM -0800, Ankur Arora wrote:
>> The scheduling policy for RESCHED_lazy (TIF_NEED_RESCHED_LAZY) is
>> to let anything running in the kernel run to completion.
>> Accordingly, while deciding whether to call preempt_schedule_irq()
>> narrow the check to tif_need_resched(RESCHED_eager).
>>
>> Also add a comment about why we need to check at all, given that we
>> have aleady checked the preempt_count().
>>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>> ---
>>  kernel/entry/common.c | 10 +++++++++-
>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
>> index 0d055c39690b..6433e6c77185 100644
>> --- a/kernel/entry/common.c
>> +++ b/kernel/entry/common.c
>> @@ -384,7 +384,15 @@ void irqentry_exit_cond_resched(void)
>>  		rcu_irq_exit_check_preempt();
>>  		if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
>>  			WARN_ON_ONCE(!on_thread_stack());
>> -		if (need_resched())
>> +
>> +		/*
>> +		 * If the scheduler really wants us to preempt while returning
>> +		 * to kernel, it would set TIF_NEED_RESCHED.
>> +		 * On some archs the flag gets folded in preempt_count, and
>> +		 * thus would be covered in the conditional above, but not all
>> +		 * archs do that, so check explicitly.
>> +		 */
>> +		if (tif_need_resched(RESCHED_eager))
>>  			preempt_schedule_irq();
>
> See, I'm reading this like if we're eager to preempt, but then it's not
> actually eager at all and only wants to preempt when forced.
>
> This naming sucks...

Yeah, it reads like it's trying to say something when it is just trying to
check a bit.

Does the new one read better?

--
ankur