Message-ID: <033f8885-9c0e-4c5a-a272-baf48807dc5d@arm.com>
Date: Wed, 25 Sep 2024 10:56:01 +0100
From: Steven Price <steven.price@....com>
To: Adrián Larumbe <adrian.larumbe@...labora.com>
Cc: Boris Brezillon <boris.brezillon@...labora.com>,
 Liviu Dudau <liviu.dudau@....com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Sumit Semwal <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>,
 kernel@...labora.com, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
 linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v6 1/5] drm/panthor: introduce job cycle and timestamp
 accounting

On 23/09/2024 21:43, Adrián Larumbe wrote:
> Hi Steve,
> 
> On 23.09.2024 09:55, Steven Price wrote:
>> On 20/09/2024 23:36, Adrián Larumbe wrote:
>>> Hi Steve, thanks for the review.
>>
>> Hi Adrián,
>>
>>> I've applied all of your suggestions for the next patch series revision, so I'll
>>> only be answering to your question about the calc_profiling_ringbuf_num_slots
>>> function further down below.
>>>
>>
>> [...]
>>
>>>>> @@ -3003,6 +3190,34 @@ static const struct drm_sched_backend_ops panthor_queue_sched_ops = {
>>>>>  	.free_job = queue_free_job,
>>>>>  };
>>>>>  
>>>>> +static u32 calc_profiling_ringbuf_num_slots(struct panthor_device *ptdev,
>>>>> +				       u32 cs_ringbuf_size)
>>>>> +{
>>>>> +	u32 min_profiled_job_instrs = U32_MAX;
>>>>> +	u32 last_flag = fls(PANTHOR_DEVICE_PROFILING_ALL);
>>>>> +
>>>>> +	/*
>>>>> +	 * We want to calculate the minimum size of a profiled job's CS,
>>>>> +	 * because since they need additional instructions for the sampling
>>>>> +	 * of performance metrics, they might take up further slots in
>>>>> +	 * the queue's ringbuffer. This means we might not need as many job
>>>>> +	 * slots for keeping track of their profiling information. What we
>>>>> +	 * need is the maximum number of slots we should allocate to this end,
>>>>> +	 * which matches the maximum number of profiled jobs we can place
>>>>> +	 * simultaneously in the queue's ring buffer.
>>>>> +	 * That has to be calculated separately for every single job profiling
>>>>> +	 * flag, but not in the case job profiling is disabled, since unprofiled
>>>>> +	 * jobs don't need to keep track of this at all.
>>>>> +	 */
>>>>> +	for (u32 i = 0; i < last_flag; i++) {
>>>>> +		if (BIT(i) & PANTHOR_DEVICE_PROFILING_ALL)
>>>>> +			min_profiled_job_instrs =
>>>>> +				min(min_profiled_job_instrs, calc_job_credits(BIT(i)));
>>>>> +	}
>>>>> +
>>>>> +	return DIV_ROUND_UP(cs_ringbuf_size, min_profiled_job_instrs * sizeof(u64));
>>>>> +}
>>>>
>>>> I may be missing something, but is there a situation where this is
>>>> different to calc_job_credits(0)? AFAICT the infrastructure you've added
>>>> can only add extra instructions to the no-flags case - whereas this
>>>> implies you're thinking that instructions may also be removed (or replaced).
>>>>
>>>> Steve
>>>
>>> Since we create a separate kernel BO to hold the profiling information slot, we
>>> need one that would be able to accomodate as many slots as the maximum number of
>>> profiled jobs we can insert simultaneously into the queue's ring buffer. Because
>>> profiled jobs always take more instructions than unprofiled ones, then we would
>>> usually need fewer slots than the number of unprofiled jobs we could insert at
>>> once in the ring buffer.
>>>
>>> Because we represent profiling metrics with a bit mask, then we need to test the
>>> size of the CS for every single metric enabled in isolation, since enabling more
>>> than one will always mean a bigger CS, and therefore fewer jobs tracked at once
>>> in the queue's ring buffer.
>>>
>>> In our case, calling calc_job_credits(0) would simply tell us the number of
>>> instructions we need for a normal job with no profiled features enabled, which
>>> would always requiere less instructions than profiled ones, and therefore more
>>> slots in the profiling info kernel BO. But we don't need to keep track of
>>> profiling numbers for unprofiled jobs, so there's no point in calculating this
>>> number.
>>>
>>> At first I was simply allocating a profiling info kernel BO as big as the number
>>> of simultaneous unprofiled job slots in the ring queue, but Boris pointed out
>>> that since queue ringbuffers can be as big as 2GiB, a lot of this memory would
>>> be wasted, since profiled jobs always require more slots because they hold more
>>> instructions, so fewer profiling slots in said kernel BO.
>>>
>>> The value of this approach will eventually manifest if we decided to keep track of
>>> more profiling metrics, since this code won't have to change at all, other than
>>> adding new profiling flags in the panthor_device_profiling_flags enum.
>>
>> Thanks for the detailed explanation. I think what I was missing is that
>> the loop is checking each bit flag independently and *not* checking
>> calc_job_credits(0).
>>
>> The check for (BIT(i) & PANTHOR_DEVICE_PROFILING_ALL) is probably what
>> confused me - that should be completely redundant. Or at least we need
>> something more intelligent if we have profiling bits which are not
>> mutually compatible.
> 
> I thought of an alternative that would only test bits that are actually part of
> the mask:
> 
> static u32 calc_profiling_ringbuf_num_slots(struct panthor_device *ptdev,
> 				       u32 cs_ringbuf_size)
> {
> 	u32 min_profiled_job_instrs = U32_MAX;
> 	u32 profiling_mask = PANTHOR_DEVICE_PROFILING_ALL;
> 
> 	while (profiling_mask) {
> 		u32 i = ffs(profiling_mask) - 1;
> 		profiling_mask &= ~BIT(i);
> 		min_profiled_job_instrs =
> 			min(min_profiled_job_instrs, calc_job_credits(BIT(i)));
> 	}
> 
> 	return DIV_ROUND_UP(cs_ringbuf_size, min_profiled_job_instrs * sizeof(u64));
> }
> 
> However, I don't think this would be more efficient, because ffs() is probably
> fetching the first set bit by performing register shifts, and I guess this would
> take somewhat longer than iterating over every single bit from the last one,
> even if also matching them against the whole mask, just in case in future
> additions of performance metrics we decide to leave some of the lower
> significance bits untouched.

Efficiency isn't very important here - we're not on a fast path, so it's
more about ensuring the code is readable. I don't think the above is
more readable then the original for loop.

> Regarding your question about mutual compatibility, I don't think that is an
> issue here, because we're testing bits in isolation. If in the future we find
> out that some of the values we're profiling cannot be sampled at once, we can
> add that logic to the sysfs knob handler, to make sure UM cannot set forbidden
> profiling masks.

My comment about compatibility is because in the original above you were
calculating the top bit of PANTHOR_DEVICE_PROFILING_ALL:

> u32 last_flag = fls(PANTHOR_DEVICE_PROFILING_ALL);

then looping between 0 and that bit:

> for (u32 i = 0; i < last_flag; i++) {

So the test:

> if (BIT(i) & PANTHOR_DEVICE_PROFILING_ALL)

would only fail if PANTHOR_DEVICE_PROFILING_ALL had gaps in the bits
that it set. The only reason I can think for that to be true in the
future is if there is some sort of incompatibility - e.g. maybe there's
an old and new way of doing some form of profiling with the old way
being kept for backwards compatibility. But I suspect if/when that is
required we'll need to revisit this function anyway. So that 'if'
statement seems completely redundant (it's trivially always true).

Steve

>> I'm also not entirely sure that the amount of RAM saved is significant,
>> but you've already written the code so we might as well have the saving ;)
> 
> I think this was more evident before Boris suggested we reduce the basic slot
> size to that of a single cache line, because then the minimum profiled job
> might've taken twice as many ringbuffer slots as a nonprofiled one. In that
> case, we would need a half as big BO for holding the sampled data (in case the
> least size profiled job CS would extend over the 16 instruction boundary).
> I still think this is a good idea so that in the future we don't need to worry
> about adjusting the code that deals with preparing the right boilerplate CS,
> since it'll only be a matter of adding new instructions inside prepare_job_instrs().
> 
>> Thanks,
>> Steve
>>
>>> Regards,
>>> Adrian
>>>
>>>>> +
>>>>>  static struct panthor_queue *
>>>>>  group_create_queue(struct panthor_group *group,
>>>>>  		   const struct drm_panthor_queue_create *args)
>>>>> @@ -3056,9 +3271,35 @@ group_create_queue(struct panthor_group *group,
>>>>>  		goto err_free_queue;
>>>>>  	}
>>>>>  
>>>>> +	queue->profiling.slot_count =
>>>>> +		calc_profiling_ringbuf_num_slots(group->ptdev, args->ringbuf_size);
>>>>> +
>>>>> +	queue->profiling.slots =
>>>>> +		panthor_kernel_bo_create(group->ptdev, group->vm,
>>>>> +					 queue->profiling.slot_count *
>>>>> +					 sizeof(struct panthor_job_profiling_data),
>>>>> +					 DRM_PANTHOR_BO_NO_MMAP,
>>>>> +					 DRM_PANTHOR_VM_BIND_OP_MAP_NOEXEC |
>>>>> +					 DRM_PANTHOR_VM_BIND_OP_MAP_UNCACHED,
>>>>> +					 PANTHOR_VM_KERNEL_AUTO_VA);
>>>>> +
>>>>> +	if (IS_ERR(queue->profiling.slots)) {
>>>>> +		ret = PTR_ERR(queue->profiling.slots);
>>>>> +		goto err_free_queue;
>>>>> +	}
>>>>> +
>>>>> +	ret = panthor_kernel_bo_vmap(queue->profiling.slots);
>>>>> +	if (ret)
>>>>> +		goto err_free_queue;
>>>>> +
>>>>> +	/*
>>>>> +	 * Credit limit argument tells us the total number of instructions
>>>>> +	 * across all CS slots in the ringbuffer, with some jobs requiring
>>>>> +	 * twice as many as others, depending on their profiling status.
>>>>> +	 */
>>>>>  	ret = drm_sched_init(&queue->scheduler, &panthor_queue_sched_ops,
>>>>>  			     group->ptdev->scheduler->wq, 1,
>>>>> -			     args->ringbuf_size / (NUM_INSTRS_PER_SLOT * sizeof(u64)),
>>>>> +			     args->ringbuf_size / sizeof(u64),
>>>>>  			     0, msecs_to_jiffies(JOB_TIMEOUT_MS),
>>>>>  			     group->ptdev->reset.wq,
>>>>>  			     NULL, "panthor-queue", group->ptdev->base.dev);
>>>>> @@ -3354,6 +3595,7 @@ panthor_job_create(struct panthor_file *pfile,
>>>>>  {
>>>>>  	struct panthor_group_pool *gpool = pfile->groups;
>>>>>  	struct panthor_job *job;
>>>>> +	u32 credits;
>>>>>  	int ret;
>>>>>  
>>>>>  	if (qsubmit->pad)
>>>>> @@ -3407,9 +3649,16 @@ panthor_job_create(struct panthor_file *pfile,
>>>>>  		}
>>>>>  	}
>>>>>  
>>>>> +	job->profiling.mask = pfile->ptdev->profile_mask;
>>>>> +	credits = calc_job_credits(job->profiling.mask);
>>>>> +	if (credits == 0) {
>>>>> +		ret = -EINVAL;
>>>>> +		goto err_put_job;
>>>>> +	}
>>>>> +
>>>>>  	ret = drm_sched_job_init(&job->base,
>>>>>  				 &job->group->queues[job->queue_idx]->entity,
>>>>> -				 1, job->group);
>>>>> +				 credits, job->group);
>>>>>  	if (ret)
>>>>>  		goto err_put_job;
>>>>>  
>>>
> 
> 
> Adrian Larumbe

Powered by blists - more mailing lists