[Intel-gfx] [PATCH v2] drm/i915: Reduce context HW ID lifetime

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-09-05 10:49:02)
> 
> On 04/09/2018 16:31, Chris Wilson wrote:
> > Future gen reduce the number of bits we will have available to
> > differentiate between contexts, so reduce the lifetime of the ID
> > assignment from that of the context to its current active cycle (i.e.
> > only while it is pinned for use by the HW, will it have a constant ID).
> > This means that instead of a max of 2k allocated contexts (worst case
> > before fun with bit twiddling), we instead have a limit of 2k in flight
> > contexts (minus a few that have been pinned by the kernel or by perf).
> > 
> > To reduce the number of contexts id we require, we allocate a context id
> > on first and mark it as pinned for as long as the GEM context itself is,
> > that is we keep it pinned it while active on each engine. If we exhaust
> > our context id space, then we try to reclaim an id from an idle context.
> > In the extreme case where all context ids are pinned by active contexts,
> > we force the system to idle in order to recover ids.
> > 
> > We cannot reduce the scope of an HW-ID to an engine (allowing the same
> > gem_context to have different ids on each engine) as in the future we
> > will need to preassign an id before we know which engine the
> > context is being executed on.
> > 
> > v2: Improved commentary (Tvrtko) [I tried at least]
> > 
> > References: https://bugs.freedesktop.org/show_bug.cgi?id=107788
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> > Cc: Michel Thierry <michel.thierry at intel.com>
> > Cc: Michal Wajdeczko <michal.wajdeczko at intel.com>
> > Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_debugfs.c           |   5 +-
> >   drivers/gpu/drm/i915/i915_drv.h               |   2 +
> >   drivers/gpu/drm/i915/i915_gem_context.c       | 222 +++++++++++++-----
> >   drivers/gpu/drm/i915/i915_gem_context.h       |  23 ++
> >   drivers/gpu/drm/i915/intel_lrc.c              |   8 +
> >   drivers/gpu/drm/i915/selftests/mock_context.c |  11 +-
> >   6 files changed, 201 insertions(+), 70 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> > index 4ad0e2ed8610..1f7051e97afb 100644
> > --- a/drivers/gpu/drm/i915/i915_debugfs.c
> > +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> > @@ -1953,7 +1953,10 @@ static int i915_context_status(struct seq_file *m, void *unused)
> >               return ret;
> >   
> >       list_for_each_entry(ctx, &dev_priv->contexts.list, link) {
> > -             seq_printf(m, "HW context %u ", ctx->hw_id);
> > +             seq_puts(m, "HW context ");
> > +             if (!list_empty(&ctx->hw_id_link))
> > +                     seq_printf(m, "%x [pin %u]", ctx->hw_id,
> > +                                atomic_read(&ctx->hw_id_pin_count));
> 
> Do you want to put some marker for the unallocated case here?

I was content with absence of marker as indicating it has never had an
id, or it had been revoked.

Who reads this file anyway? There's not one igt where I've thought this
would be useful debug info. Maybe I'm wrong...
-Chris