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@xxxxxxxxx>
> > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> > ---
> > 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);
> > }
>
