Re: [PATCH] io_uring: Use io_schedule* in cqring wait

[Jens Axboe <axboe@kernel.dk>]

On 7/24/23 10:16?AM, Andres Freund wrote:
> Hi,
> 
> On 2023-07-24 09:48:58 -0600, Jens Axboe wrote:
>> On 7/24/23 9:35?AM, Phil Elwell wrote:
>>> Hi Andres,
>>>
>>> With this commit applied to the 6.1 and later kernels (others not
>>> tested) the iowait time ("wa" field in top) in an ARM64 build running
>>> on a 4 core CPU (a Raspberry Pi 4 B) increases to 25%, as if one core
>>> is permanently blocked on I/O. The change can be observed after
>>> installing mariadb-server (no configuration or use is required). After
>>> reverting just this commit, "wa" drops to zero again.
>>
>> There are a few other threads on this...
>>
>>> I can believe that this change hasn't negatively affected performance,
>>> but the result is misleading. I also think it's pushing the boundaries
>>> of what a back-port to stable should do.
> 
> FWIW, I think this partially just mpstat reporting something quite bogus. It
> makes no sense to say that a cpu is 100% busy waiting for IO, when the one
> process is doing IO is just waiting.

Indeed... It really just means it's spending 100% of its time _waiting_
on IO, not that it's doing anything. This is largely to save myself from
future emails on this subject, saving my own time.

>> +static bool current_pending_io(void)
>> +{
>> +	struct io_uring_task *tctx = current->io_uring;
>> +
>> +	if (!tctx)
>> +		return false;
>> +	return percpu_counter_read_positive(&tctx->inflight);
>> +}
>> +
>>  /* when returns >0, the caller should retry */
>>  static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
>>  					  struct io_wait_queue *iowq)
>>  {
>> -	int token, ret;
>> +	int io_wait, ret;
>>  
>>  	if (unlikely(READ_ONCE(ctx->check_cq)))
>>  		return 1;
>> @@ -2511,17 +2520,19 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
>>  		return 0;
>>  
>>  	/*
>> -	 * Use io_schedule_prepare/finish, so cpufreq can take into account
>> -	 * that the task is waiting for IO - turns out to be important for low
>> -	 * QD IO.
>> +	 * Mark us as being in io_wait if we have pending requests, so cpufreq
>> +	 * can take into account that the task is waiting for IO - turns out
>> +	 * to be important for low QD IO.
>>  	 */
>> -	token = io_schedule_prepare();
>> +	io_wait = current->in_iowait;
> 
> I don't know the kernel "rules" around this, but ->in_iowait is only
> modified in kernel/sched, so it seemed a tad "unfriendly" to scribble
> on it here...

It's either that or add new helpers for this, at least for the initial
one. Calling blk_flush_plug() (and with async == true, no less) is not
something we need or want to do.

So we could add an io_schedule_prepare_noflush() for this, but also
seems silly to add a single use helper for that imho.

> Building a kernel to test with the patch applied, will reboot into it
> once the call I am on has finished. Unfortunately the performance
> difference didn't reproduce nicely in VM...

Thanks!

-- 
Jens Axboe