Re: [Intel-gfx] [PATCH] drm/i915/guc: Use lockless list for destroyed contexts

[Matthew Brost <matthew.brost@intel.com>]

On Wed, Dec 22, 2021 at 04:48:36PM -0800, John Harrison wrote:
> On 12/22/2021 15:29, Matthew Brost wrote:
> > Use a lockless list structure for destroyed contexts to avoid hammering
> > on global submission spin lock.
> I thought the guidance was that lockless anything without an explanation
> longer than War And Peace comes with an automatic termination penalty?
> 

I was thinking that was for custom lockless algorithms not using core
uAPIs. If this is really concern I could protect the llist_del_all by a
lock but the doc explicitly says how I'm using this uAPI is safe without
a lock. 

> Also, I thought the simple suggestion was to just move the entire list
> sideways under the existing lock and then loop through the local list safely
> without requiring locks because it is now local only.
> 

That's basically what this uAPI does in a few simple calls rather than
our own algorithm to move to a new list.

Matt

> John.
> 
> 
> > 
> > Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >   drivers/gpu/drm/i915/gt/intel_context.c       |  2 -
> >   drivers/gpu/drm/i915/gt/intel_context_types.h |  3 +-
> >   drivers/gpu/drm/i915/gt/uc/intel_guc.h        |  3 +-
> >   .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 44 +++++--------------
> >   4 files changed, 16 insertions(+), 36 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> > index 5d0ec7c49b6a..4aacb4b0418d 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_context.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> > @@ -403,8 +403,6 @@ intel_context_init(struct intel_context *ce, struct intel_engine_cs *engine)
> >   	ce->guc_id.id = GUC_INVALID_LRC_ID;
> >   	INIT_LIST_HEAD(&ce->guc_id.link);
> > -	INIT_LIST_HEAD(&ce->destroyed_link);
> > -
> >   	INIT_LIST_HEAD(&ce->parallel.child_list);
> >   	/*
> > diff --git a/drivers/gpu/drm/i915/gt/intel_context_types.h b/drivers/gpu/drm/i915/gt/intel_context_types.h
> > index 30cd81ad8911..4532d43ec9c0 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_context_types.h
> > +++ b/drivers/gpu/drm/i915/gt/intel_context_types.h
> > @@ -9,6 +9,7 @@
> >   #include <linux/average.h>
> >   #include <linux/kref.h>
> >   #include <linux/list.h>
> > +#include <linux/llist.h>
> >   #include <linux/mutex.h>
> >   #include <linux/types.h>
> > @@ -224,7 +225,7 @@ struct intel_context {
> >   	 * list when context is pending to be destroyed (deregistered with the
> >   	 * GuC), protected by guc->submission_state.lock
> >   	 */
> > -	struct list_head destroyed_link;
> > +	struct llist_node destroyed_link;
> >   	/** @parallel: sub-structure for parallel submission members */
> >   	struct {
> > diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc.h b/drivers/gpu/drm/i915/gt/uc/intel_guc.h
> > index f9240d4baa69..705085058411 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc.h
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc.h
> > @@ -8,6 +8,7 @@
> >   #include <linux/xarray.h>
> >   #include <linux/delay.h>
> > +#include <linux/llist.h>
> >   #include "intel_uncore.h"
> >   #include "intel_guc_fw.h"
> > @@ -112,7 +113,7 @@ struct intel_guc {
> >   		 * @destroyed_contexts: list of contexts waiting to be destroyed
> >   		 * (deregistered with the GuC)
> >   		 */
> > -		struct list_head destroyed_contexts;
> > +		struct llist_head destroyed_contexts;
> >   		/**
> >   		 * @destroyed_worker: worker to deregister contexts, need as we
> >   		 * need to take a GT PM reference and can't from destroy
> > diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > index 0a03a30e4c6d..6f7643edc139 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > @@ -1771,7 +1771,7 @@ int intel_guc_submission_init(struct intel_guc *guc)
> >   	spin_lock_init(&guc->submission_state.lock);
> >   	INIT_LIST_HEAD(&guc->submission_state.guc_id_list);
> >   	ida_init(&guc->submission_state.guc_ids);
> > -	INIT_LIST_HEAD(&guc->submission_state.destroyed_contexts);
> > +	init_llist_head(&guc->submission_state.destroyed_contexts);
> >   	INIT_WORK(&guc->submission_state.destroyed_worker,
> >   		  destroyed_worker_func);
> > @@ -2696,26 +2696,18 @@ static void __guc_context_destroy(struct intel_context *ce)
> >   	}
> >   }
> > +#define take_destroyed_contexts(guc) \
> > +	llist_del_all(&guc->submission_state.destroyed_contexts)
> > +
> >   static void guc_flush_destroyed_contexts(struct intel_guc *guc)
> >   {
> > -	struct intel_context *ce;
> > -	unsigned long flags;
> > +	struct intel_context *ce, *cn;
> >   	GEM_BUG_ON(!submission_disabled(guc) &&
> >   		   guc_submission_initialized(guc));
> > -	while (!list_empty(&guc->submission_state.destroyed_contexts)) {
> > -		spin_lock_irqsave(&guc->submission_state.lock, flags);
> > -		ce = list_first_entry_or_null(&guc->submission_state.destroyed_contexts,
> > -					      struct intel_context,
> > -					      destroyed_link);
> > -		if (ce)
> > -			list_del_init(&ce->destroyed_link);
> > -		spin_unlock_irqrestore(&guc->submission_state.lock, flags);
> > -
> > -		if (!ce)
> > -			break;
> > -
> > +	llist_for_each_entry_safe(ce, cn, take_destroyed_contexts(guc),
> > +				 destroyed_link) {
> >   		release_guc_id(guc, ce);
> >   		__guc_context_destroy(ce);
> >   	}
> > @@ -2723,23 +2715,11 @@ static void guc_flush_destroyed_contexts(struct intel_guc *guc)
> >   static void deregister_destroyed_contexts(struct intel_guc *guc)
> >   {
> > -	struct intel_context *ce;
> > -	unsigned long flags;
> > -
> > -	while (!list_empty(&guc->submission_state.destroyed_contexts)) {
> > -		spin_lock_irqsave(&guc->submission_state.lock, flags);
> > -		ce = list_first_entry_or_null(&guc->submission_state.destroyed_contexts,
> > -					      struct intel_context,
> > -					      destroyed_link);
> > -		if (ce)
> > -			list_del_init(&ce->destroyed_link);
> > -		spin_unlock_irqrestore(&guc->submission_state.lock, flags);
> > -
> > -		if (!ce)
> > -			break;
> > +	struct intel_context *ce, *cn;
> > +	llist_for_each_entry_safe(ce, cn, take_destroyed_contexts(guc),
> > +				 destroyed_link)
> >   		guc_lrc_desc_unpin(ce);
> > -	}
> >   }
> >   static void destroyed_worker_func(struct work_struct *w)
> > @@ -2771,8 +2751,8 @@ static void guc_context_destroy(struct kref *kref)
> >   	if (likely(!destroy)) {
> >   		if (!list_empty(&ce->guc_id.link))
> >   			list_del_init(&ce->guc_id.link);
> > -		list_add_tail(&ce->destroyed_link,
> > -			      &guc->submission_state.destroyed_contexts);
> > +		llist_add(&ce->destroyed_link,
> > +			  &guc->submission_state.destroyed_contexts);
> >   	} else {
> >   		__release_guc_id(guc, ce);
> >   	}
>