Re: [PATCH v3 4/7] drm/i915: Switch to fdinfo helper

From: Daniel Vetter
Date: Wed Apr 12 2023 - 09:51:31 EST

On Wed, Apr 12, 2023 at 01:32:43PM +0100, Tvrtko Ursulin wrote:
>
> On 11/04/2023 23:56, Rob Clark wrote:
> > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >
> > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/i915_driver.c | 3 ++-
> > drivers/gpu/drm/i915/i915_drm_client.c | 18 +++++-------------
> > drivers/gpu/drm/i915/i915_drm_client.h | 2 +-
> > 3 files changed, 8 insertions(+), 15 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_driver.c b/drivers/gpu/drm/i915/i915_driver.c
> > index db7a86def7e2..37eacaa3064b 100644
> > --- a/drivers/gpu/drm/i915/i915_driver.c
> > +++ b/drivers/gpu/drm/i915/i915_driver.c
> > @@ -1696,7 +1696,7 @@ static const struct file_operations i915_driver_fops = {
> > .compat_ioctl = i915_ioc32_compat_ioctl,
> > .llseek = noop_llseek,
> > #ifdef CONFIG_PROC_FS
> > - .show_fdinfo = i915_drm_client_fdinfo,
> > + .show_fdinfo = drm_fop_show_fdinfo,
> > #endif
> > };
> > @@ -1796,6 +1796,7 @@ static const struct drm_driver i915_drm_driver = {
> > .open = i915_driver_open,
> > .lastclose = i915_driver_lastclose,
> > .postclose = i915_driver_postclose,
> > + .show_fdinfo = i915_drm_client_fdinfo,
> > .prime_handle_to_fd = drm_gem_prime_handle_to_fd,
> > .prime_fd_to_handle = drm_gem_prime_fd_to_handle,
> > diff --git a/drivers/gpu/drm/i915/i915_drm_client.c b/drivers/gpu/drm/i915/i915_drm_client.c
> > index b09d1d386574..4a77e5e47f79 100644
> > --- a/drivers/gpu/drm/i915/i915_drm_client.c
> > +++ b/drivers/gpu/drm/i915/i915_drm_client.c
> > @@ -101,7 +101,7 @@ static u64 busy_add(struct i915_gem_context *ctx, unsigned int class)
> > }
> > static void
> > -show_client_class(struct seq_file *m,
> > +show_client_class(struct drm_printer *p,
> > struct i915_drm_client *client,
> > unsigned int class)
> > {
> > @@ -117,22 +117,20 @@ show_client_class(struct seq_file *m,
> > rcu_read_unlock();
> > if (capacity)
> > - seq_printf(m, "drm-engine-%s:\t%llu ns\n",
> > + drm_printf(p, "drm-engine-%s:\t%llu ns\n",
> > uabi_class_names[class], total);
> > if (capacity > 1)
> > - seq_printf(m, "drm-engine-capacity-%s:\t%u\n",
> > + drm_printf(p, "drm-engine-capacity-%s:\t%u\n",
> > uabi_class_names[class],
> > capacity);
> > }
> > -void i915_drm_client_fdinfo(struct seq_file *m, struct file *f)
> > +void i915_drm_client_fdinfo(struct drm_printer *p, struct drm_file *file)
> > {
> > - struct drm_file *file = f->private_data;
> > struct drm_i915_file_private *file_priv = file->driver_priv;
> > struct drm_i915_private *i915 = file_priv->dev_priv;
> > struct i915_drm_client *client = file_priv->client;
> > - struct pci_dev *pdev = to_pci_dev(i915->drm.dev);
> > unsigned int i;
> > /*
> > @@ -141,12 +139,6 @@ void i915_drm_client_fdinfo(struct seq_file *m, struct file *f)
> > * ******************************************************************
> > */
> > - seq_printf(m, "drm-driver:\t%s\n", i915->drm.driver->name);
> > - seq_printf(m, "drm-pdev:\t%04x:%02x:%02x.%d\n",
> > - pci_domain_nr(pdev->bus), pdev->bus->number,
> > - PCI_SLOT(pdev->devfn), PCI_FUNC(pdev->devfn));
> > - seq_printf(m, "drm-client-id:\t%u\n", client->id);
>
> As mentioned in my reply to the cover letter, I think the i915
> implementation is the right one. At least the semantics of it.
>
> Granted it is a super set of the minimum required as documented by
> drm-usage-stats.rst - not only 1:1 to current instances of struct file, but
> also avoids immediate id recycling.
>
> Former could perhaps be achieved with a simple pointer hash, but latter
> helps userspace detect when a client has exited and id re-allocated to a new
> client within a single scanning period.
>
> Without this I don't think userspace can implement a fail safe method of
> detecting which clients are new ones and so wouldn't be able to track
> history correctly.
>
> I think we should rather extend the documented contract to include the
> cyclical property than settle for a weaker common implementation.

atomic64_t never wraps, so you don't have any recycling issues?

The other piece and imo much more important is that I really don't want
the i915_drm_client design to spread, it conceptually makes no sense.
drm_file is the uapi object, once that's gone userspace will never be able
to look at anything, having a separate free-standing object that's
essentially always dead is backwards.

I went a bit more in-depth in a different thread on scheduler fd_info
stats, but essentially fd_info needs to pull stats, you should never push
stats towards the drm_file (or i915_drm_client). That avoids all the
refcounting issues and rcu needs and everything else like that.

Maybe you want to jump into that thread:
https://lore.kernel.org/dri-devel/CAKMK7uE=m3sSTQrLCeDg0vG8viODOecUsYDK1oC++f5pQi0e8Q@xxxxxxxxxxxxxx/

So retiring i915_drm_client infrastructure is the right direction I think.
-Daniel

> Regards,
>
> Tvrtko
>
> > -
> > /*
> > * Temporarily skip showing client engine information with GuC submission till
> > * fetching engine busyness is implemented in the GuC submission backend
> > @@ -155,6 +147,6 @@ void i915_drm_client_fdinfo(struct seq_file *m, struct file *f)
> > return;
> > for (i = 0; i < ARRAY_SIZE(uabi_class_names); i++)
> > - show_client_class(m, client, i);
> > + show_client_class(p, client, i);
> > }
> > #endif
> > diff --git a/drivers/gpu/drm/i915/i915_drm_client.h b/drivers/gpu/drm/i915/i915_drm_client.h
> > index 69496af996d9..ef85fef45de5 100644
> > --- a/drivers/gpu/drm/i915/i915_drm_client.h
> > +++ b/drivers/gpu/drm/i915/i915_drm_client.h
> > @@ -60,7 +60,7 @@ static inline void i915_drm_client_put(struct i915_drm_client *client)
> > struct i915_drm_client *i915_drm_client_add(struct i915_drm_clients *clients);
> > #ifdef CONFIG_PROC_FS
> > -void i915_drm_client_fdinfo(struct seq_file *m, struct file *f);
> > +void i915_drm_client_fdinfo(struct drm_printer *p, struct drm_file *file);
> > #endif
> > void i915_drm_clients_fini(struct i915_drm_clients *clients);

--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch