Re: [Intel-gfx] [PATCH] drm/i915: Ditch i915 globals shrink infrastructure

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 22/07/2021 11:16, Daniel Vetter wrote:
> On Thu, Jul 22, 2021 at 11:02:55AM +0100, Tvrtko Ursulin wrote:
>> On 21/07/2021 19:32, Daniel Vetter wrote:
>>> This essentially reverts
>>>
>>> commit 84a1074920523430f9dc30ff907f4801b4820072
>>> Author: Chris Wilson <chris@chris-wilson.co.uk>
>>> Date:   Wed Jan 24 11:36:08 2018 +0000
>>>
>>>       drm/i915: Shrink the GEM kmem_caches upon idling
>>>
>>> mm/vmscan.c:do_shrink_slab() is a thing, if there's an issue with it
>>> then we need to fix that there, not hand-roll our own slab shrinking
>>> code in i915.
>>
>> This is somewhat incomplete statement which ignores a couple of angles so I
>> wish there was a bit more time to respond before steam rolling it in. :(
>>
>> The removed code was not a hand rolled shrinker, but about managing slab
>> sizes in face of bursty workloads. Core code does not know when i915 is
>> active and when it is idle, so calling kmem_cache_shrink() after going idle
>> wass supposed to help with house keeping by doing house keeping work outside
>> of the latency sensitive phase.
>>
>> To "fix" (improve really) it in core as you suggest, would need some method
>> of signaling when a slab user feels is an opportunte moment to do this house
>> keeping. And kmem_cache_shrink is just that so I don't see the problem.
>>
>> Granted, argument kmem_cache_shrink is not much used is a valid one so
>> discussion overall is definitely valid. Becuase on the higher level we could
>> definitely talk about which workloads actually benefit from this code and
>> how much which probably no one knows at this point.
>>
>> But in general I think you needed to leave more time for discussion. 12
>> hours is way too short.
> 
> It's 500+ users of kmem_cache_create vs i915 doing kmem_cache_shrink. And

There are two other callers for the record. ;)

> I guarantee you there's slab users that churn through more allocations
> than we do, and are more bursty.

I wasn't disputing that.

> An extraordinary claim like this needs extraordinary evidence. And then a
> discussion with core mm/ folks so that we can figure out how to solve the
> discovered problem best for the other 500+ users of slabs in-tree, so that
> everyone benefits. Not just i915 gpu workloads.

Yep, not disputing that either. Noticed I wrote it was a valid argument?

But discussion with mm folks could also have happened before you steam 
rolled the "revert" in though. Perhaps tey would have said 
kmem_cache_shrink is the way. Or maybe it isn't. Or maybe they would 
have said meh. I just don't see how the rush was justified given the 
code in question.

Regards,

Tvrtko

> -Daniel
> 
>>> Noticed while reviewing a patch set from Jason to fix up some issues
>>> in our i915_init() and i915_exit() module load/cleanup code. Now that
>>> i915_globals.c isn't any different than normal init/exit functions, we
>>> should convert them over to one unified table and remove
>>> i915_globals.[hc] entirely.
>>>
>>> Cc: David Airlie <airlied@linux.ie>
>>> Cc: Jason Ekstrand <jason@jlekstrand.net>
>>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
>>> ---
>>>    drivers/gpu/drm/i915/gem/i915_gem_context.c |  6 --
>>>    drivers/gpu/drm/i915/gem/i915_gem_object.c  |  6 --
>>>    drivers/gpu/drm/i915/gt/intel_context.c     |  6 --
>>>    drivers/gpu/drm/i915/gt/intel_gt_pm.c       |  4 -
>>>    drivers/gpu/drm/i915/i915_active.c          |  6 --
>>>    drivers/gpu/drm/i915/i915_globals.c         | 95 ---------------------
>>>    drivers/gpu/drm/i915/i915_globals.h         |  3 -
>>>    drivers/gpu/drm/i915/i915_request.c         |  7 --
>>>    drivers/gpu/drm/i915/i915_scheduler.c       |  7 --
>>>    drivers/gpu/drm/i915/i915_vma.c             |  6 --
>>>    10 files changed, 146 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>> index 7d6f52d8a801..bf2a2319353a 100644
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>> @@ -2280,18 +2280,12 @@ i915_gem_engines_iter_next(struct i915_gem_engines_iter *it)
>>>    #include "selftests/i915_gem_context.c"
>>>    #endif
>>> -static void i915_global_gem_context_shrink(void)
>>> -{
>>> -	kmem_cache_shrink(global.slab_luts);
>>> -}
>>> -
>>>    static void i915_global_gem_context_exit(void)
>>>    {
>>>    	kmem_cache_destroy(global.slab_luts);
>>>    }
>>>    static struct i915_global_gem_context global = { {
>>> -	.shrink = i915_global_gem_context_shrink,
>>>    	.exit = i915_global_gem_context_exit,
>>>    } };
>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
>>> index 9da7b288b7ed..5c21cff33199 100644
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
>>> @@ -664,18 +664,12 @@ void i915_gem_init__objects(struct drm_i915_private *i915)
>>>    	INIT_WORK(&i915->mm.free_work, __i915_gem_free_work);
>>>    }
>>> -static void i915_global_objects_shrink(void)
>>> -{
>>> -	kmem_cache_shrink(global.slab_objects);
>>> -}
>>> -
>>>    static void i915_global_objects_exit(void)
>>>    {
>>>    	kmem_cache_destroy(global.slab_objects);
>>>    }
>>>    static struct i915_global_object global = { {
>>> -	.shrink = i915_global_objects_shrink,
>>>    	.exit = i915_global_objects_exit,
>>>    } };
>>> diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
>>> index bd63813c8a80..c1338441cc1d 100644
>>> --- a/drivers/gpu/drm/i915/gt/intel_context.c
>>> +++ b/drivers/gpu/drm/i915/gt/intel_context.c
>>> @@ -398,18 +398,12 @@ void intel_context_fini(struct intel_context *ce)
>>>    	i915_active_fini(&ce->active);
>>>    }
>>> -static void i915_global_context_shrink(void)
>>> -{
>>> -	kmem_cache_shrink(global.slab_ce);
>>> -}
>>> -
>>>    static void i915_global_context_exit(void)
>>>    {
>>>    	kmem_cache_destroy(global.slab_ce);
>>>    }
>>>    static struct i915_global_context global = { {
>>> -	.shrink = i915_global_context_shrink,
>>>    	.exit = i915_global_context_exit,
>>>    } };
>>> diff --git a/drivers/gpu/drm/i915/gt/intel_gt_pm.c b/drivers/gpu/drm/i915/gt/intel_gt_pm.c
>>> index aef3084e8b16..d86825437516 100644
>>> --- a/drivers/gpu/drm/i915/gt/intel_gt_pm.c
>>> +++ b/drivers/gpu/drm/i915/gt/intel_gt_pm.c
>>> @@ -67,8 +67,6 @@ static int __gt_unpark(struct intel_wakeref *wf)
>>>    	GT_TRACE(gt, "\n");
>>> -	i915_globals_unpark();
>>> -
>>>    	/*
>>>    	 * It seems that the DMC likes to transition between the DC states a lot
>>>    	 * when there are no connected displays (no active power domains) during
>>> @@ -116,8 +114,6 @@ static int __gt_park(struct intel_wakeref *wf)
>>>    	GEM_BUG_ON(!wakeref);
>>>    	intel_display_power_put_async(i915, POWER_DOMAIN_GT_IRQ, wakeref);
>>> -	i915_globals_park();
>>> -
>>>    	return 0;
>>>    }
>>> diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
>>> index b1aa1c482c32..91723123ae9f 100644
>>> --- a/drivers/gpu/drm/i915/i915_active.c
>>> +++ b/drivers/gpu/drm/i915/i915_active.c
>>> @@ -1176,18 +1176,12 @@ struct i915_active *i915_active_create(void)
>>>    #include "selftests/i915_active.c"
>>>    #endif
>>> -static void i915_global_active_shrink(void)
>>> -{
>>> -	kmem_cache_shrink(global.slab_cache);
>>> -}
>>> -
>>>    static void i915_global_active_exit(void)
>>>    {
>>>    	kmem_cache_destroy(global.slab_cache);
>>>    }
>>>    static struct i915_global_active global = { {
>>> -	.shrink = i915_global_active_shrink,
>>>    	.exit = i915_global_active_exit,
>>>    } };
>>> diff --git a/drivers/gpu/drm/i915/i915_globals.c b/drivers/gpu/drm/i915/i915_globals.c
>>> index 77f1911c463b..7fe2e503897b 100644
>>> --- a/drivers/gpu/drm/i915/i915_globals.c
>>> +++ b/drivers/gpu/drm/i915/i915_globals.c
>>> @@ -17,61 +17,8 @@
>>>    static LIST_HEAD(globals);
>>> -static atomic_t active;
>>> -static atomic_t epoch;
>>> -static struct park_work {
>>> -	struct delayed_work work;
>>> -	struct rcu_head rcu;
>>> -	unsigned long flags;
>>> -#define PENDING 0
>>> -	int epoch;
>>> -} park;
>>> -
>>> -static void i915_globals_shrink(void)
>>> -{
>>> -	struct i915_global *global;
>>> -
>>> -	/*
>>> -	 * kmem_cache_shrink() discards empty slabs and reorders partially
>>> -	 * filled slabs to prioritise allocating from the mostly full slabs,
>>> -	 * with the aim of reducing fragmentation.
>>> -	 */
>>> -	list_for_each_entry(global, &globals, link)
>>> -		global->shrink();
>>> -}
>>> -
>>> -static void __i915_globals_grace(struct rcu_head *rcu)
>>> -{
>>> -	/* Ratelimit parking as shrinking is quite slow */
>>> -	schedule_delayed_work(&park.work, round_jiffies_up_relative(2 * HZ));
>>> -}
>>> -
>>> -static void __i915_globals_queue_rcu(void)
>>> -{
>>> -	park.epoch = atomic_inc_return(&epoch);
>>> -	if (!atomic_read(&active)) {
>>> -		init_rcu_head(&park.rcu);
>>> -		call_rcu(&park.rcu, __i915_globals_grace);
>>> -	}
>>> -}
>>> -
>>> -static void __i915_globals_park(struct work_struct *work)
>>> -{
>>> -	destroy_rcu_head(&park.rcu);
>>> -
>>> -	/* Confirm nothing woke up in the last grace period */
>>> -	if (park.epoch != atomic_read(&epoch)) {
>>> -		__i915_globals_queue_rcu();
>>> -		return;
>>> -	}
>>> -
>>> -	clear_bit(PENDING, &park.flags);
>>> -	i915_globals_shrink();
>>> -}
>>> -
>>>    void __init i915_global_register(struct i915_global *global)
>>>    {
>>> -	GEM_BUG_ON(!global->shrink);
>>>    	GEM_BUG_ON(!global->exit);
>>>    	list_add_tail(&global->link, &globals);
>>> @@ -109,52 +56,10 @@ int __init i915_globals_init(void)
>>>    		}
>>>    	}
>>> -	INIT_DELAYED_WORK(&park.work, __i915_globals_park);
>>>    	return 0;
>>>    }
>>> -void i915_globals_park(void)
>>> -{
>>> -	/*
>>> -	 * Defer shrinking the global slab caches (and other work) until
>>> -	 * after a RCU grace period has completed with no activity. This
>>> -	 * is to try and reduce the latency impact on the consumers caused
>>> -	 * by us shrinking the caches the same time as they are trying to
>>> -	 * allocate, with the assumption being that if we idle long enough
>>> -	 * for an RCU grace period to elapse since the last use, it is likely
>>> -	 * to be longer until we need the caches again.
>>> -	 */
>>> -	if (!atomic_dec_and_test(&active))
>>> -		return;
>>> -
>>> -	/* Queue cleanup after the next RCU grace period has freed slabs */
>>> -	if (!test_and_set_bit(PENDING, &park.flags))
>>> -		__i915_globals_queue_rcu();
>>> -}
>>> -
>>> -void i915_globals_unpark(void)
>>> -{
>>> -	atomic_inc(&epoch);
>>> -	atomic_inc(&active);
>>> -}
>>> -
>>> -static void __exit __i915_globals_flush(void)
>>> -{
>>> -	atomic_inc(&active); /* skip shrinking */
>>> -
>>> -	rcu_barrier(); /* wait for the work to be queued */
>>> -	flush_delayed_work(&park.work);
>>> -
>>> -	atomic_dec(&active);
>>> -}
>>> -
>>>    void __exit i915_globals_exit(void)
>>>    {
>>> -	GEM_BUG_ON(atomic_read(&active));
>>> -
>>> -	__i915_globals_flush();
>>>    	__i915_globals_cleanup();
>>> -
>>> -	/* And ensure that our DESTROY_BY_RCU slabs are truly destroyed */
>>> -	rcu_barrier();
>>>    }
>>> diff --git a/drivers/gpu/drm/i915/i915_globals.h b/drivers/gpu/drm/i915/i915_globals.h
>>> index 2d199f411a4a..9e6b4fd07528 100644
>>> --- a/drivers/gpu/drm/i915/i915_globals.h
>>> +++ b/drivers/gpu/drm/i915/i915_globals.h
>>> @@ -14,15 +14,12 @@ typedef void (*i915_global_func_t)(void);
>>>    struct i915_global {
>>>    	struct list_head link;
>>> -	i915_global_func_t shrink;
>>>    	i915_global_func_t exit;
>>>    };
>>>    void i915_global_register(struct i915_global *global);
>>>    int i915_globals_init(void);
>>> -void i915_globals_park(void);
>>> -void i915_globals_unpark(void);
>>>    void i915_globals_exit(void);
>>>    /* constructors */
>>> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
>>> index 09ebea9a0090..d3de9f60e03a 100644
>>> --- a/drivers/gpu/drm/i915/i915_request.c
>>> +++ b/drivers/gpu/drm/i915/i915_request.c
>>> @@ -2077,12 +2077,6 @@ void i915_request_show(struct drm_printer *m,
>>>    #include "selftests/i915_request.c"
>>>    #endif
>>> -static void i915_global_request_shrink(void)
>>> -{
>>> -	kmem_cache_shrink(global.slab_execute_cbs);
>>> -	kmem_cache_shrink(global.slab_requests);
>>> -}
>>> -
>>>    static void i915_global_request_exit(void)
>>>    {
>>>    	kmem_cache_destroy(global.slab_execute_cbs);
>>> @@ -2090,7 +2084,6 @@ static void i915_global_request_exit(void)
>>>    }
>>>    static struct i915_global_request global = { {
>>> -	.shrink = i915_global_request_shrink,
>>>    	.exit = i915_global_request_exit,
>>>    } };
>>> diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
>>> index 3a58a9130309..561c649e59f7 100644
>>> --- a/drivers/gpu/drm/i915/i915_scheduler.c
>>> +++ b/drivers/gpu/drm/i915/i915_scheduler.c
>>> @@ -475,12 +475,6 @@ i915_sched_engine_create(unsigned int subclass)
>>>    	return sched_engine;
>>>    }
>>> -static void i915_global_scheduler_shrink(void)
>>> -{
>>> -	kmem_cache_shrink(global.slab_dependencies);
>>> -	kmem_cache_shrink(global.slab_priorities);
>>> -}
>>> -
>>>    static void i915_global_scheduler_exit(void)
>>>    {
>>>    	kmem_cache_destroy(global.slab_dependencies);
>>> @@ -488,7 +482,6 @@ static void i915_global_scheduler_exit(void)
>>>    }
>>>    static struct i915_global_scheduler global = { {
>>> -	.shrink = i915_global_scheduler_shrink,
>>>    	.exit = i915_global_scheduler_exit,
>>>    } };
>>> diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
>>> index 5b9dce0f443b..09a7c47926f7 100644
>>> --- a/drivers/gpu/drm/i915/i915_vma.c
>>> +++ b/drivers/gpu/drm/i915/i915_vma.c
>>> @@ -1414,18 +1414,12 @@ void i915_vma_make_purgeable(struct i915_vma *vma)
>>>    #include "selftests/i915_vma.c"
>>>    #endif
>>> -static void i915_global_vma_shrink(void)
>>> -{
>>> -	kmem_cache_shrink(global.slab_vmas);
>>> -}
>>> -
>>>    static void i915_global_vma_exit(void)
>>>    {
>>>    	kmem_cache_destroy(global.slab_vmas);
>>>    }
>>>    static struct i915_global_vma global = { {
>>> -	.shrink = i915_global_vma_shrink,
>>>    	.exit = i915_global_vma_exit,
>>>    } };
>>>
> 
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