Date: Wed, 1 May 2024 09:25:01 -0700
From: Rob Clark <robdclark@...il.com>
To: Lucas De Marchi <lucas.demarchi@...el.com>
Cc: Tvrtko Ursulin <tursulin@...ulin.net>, Adrián Larumbe <adrian.larumbe@...labora.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Hans de Goede <hdegoede@...hat.com>, kernel@...labora.com, 
	Boris Brezillon <boris.brezillon@...labora.com>, Christopher Healy <healych@...zon.com>, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/sysfs: Add drm class-wide attribute to get active
 device clients

On Wed, May 1, 2024 at 9:19 AM Lucas De Marchi <lucas.demarchi@...el.com> wrote:
>
> On Wed, May 01, 2024 at 04:58:05PM GMT, Tvrtko Ursulin wrote:
> >
> >Hi,
> >
> >On 24/04/2024 15:48, Adrián Larumbe wrote:
> >>Hi Tvrtko,
> >>
> >>On 15.04.2024 13:50, Tvrtko Ursulin wrote:
> >>>
> >>>On 05/04/2024 18:59, Rob Clark wrote:
> >>>>On Wed, Apr 3, 2024 at 11:37 AM Adrián Larumbe
> >>>><adrian.larumbe@...labora.com> wrote:
> >>>>>
> >>>>>Up to this day, all fdinfo-based GPU profilers must traverse the entire
> >>>>>/proc directory structure to find open DRM clients with fdinfo file
> >>>>>descriptors. This is inefficient and time-consuming.
> >>>>>
> >>>>>This patch adds a new device class attribute that will install a sysfs file
> >>>>>per DRM device, which can be queried by profilers to get a list of PIDs for
> >>>>>their open clients. This file isn't human-readable, and it's meant to be
> >>>>>queried only by GPU profilers like gputop and nvtop.
> >>>>>
> >>>>>Cc: Boris Brezillon <boris.brezillon@...labora.com>
> >>>>>Cc: Tvrtko Ursulin <tursulin@...ulin.net>
> >>>>>Cc: Christopher Healy <healych@...zon.com>
> >>>>>Signed-off-by: Adrián Larumbe <adrian.larumbe@...labora.com>
> >>>>
> >>>>It does seem like a good idea.. idk if there is some precedent to
> >>>>prefer binary vs ascii in sysfs, but having a way to avoid walking
> >>>>_all_ processes is a good idea.
> >>>
> >>>I naturally second that it is a needed feature, but I do not think binary
> >>>format is justified. AFAIR it should be used for things like hw/fw
> >>>standardised tables or firmware images, not when exporting a simple list of
> >>>PIDs. It also precludes easy shell/script access and the benefit of avoiding
> >>>parsing a short list is I suspect completely dwarfed by needing to parse all
> >>>the related fdinfo etc.
> >>
> >>I'd rather keep it as a binary file for the sake of easily parsing the number
> >>list on the client side, in gputop or nvtop. For textual access, there's already
> >>a debugfs file that presents the same information, so I thought it was best not
> >>to duplicate that functionality and restrict sysfs to serving the very specific
> >>use case of UM profilers having to access the DRM client list.
> >>
> >>I should mention I did something controversial here, which is a semantically
> >>binary attribute through the regular attribute interface. I guess if I keep it
> >>as a binary attribute in the end, I should switch over to the binary attribute
> >>API.
> >>
> >>Another reason why I implemented it as a binary file is that we can only send
> >>back at most a whole page. If a PID takes 4 bytes, that's usually 1024 clients
> >>at most, which is probably enough for any UM profiler, but will decrease even
> >>more if we turn it into an ASCII readable file.
> >
> >I'm afraid I still think there is no reason for a binary file, even
> >less so artificially limited to 1024 clients. Any consumer will have
> >to parse text fdinfo so a binary list of pids is not adding any real
> >cost.
>
> yeah, I don't really understand why you'd want the binary number that
> you'd then have to turn into a string to open the /proc/<pid>/. To me it
> sounds more like we want a text output and that output to be:
>
> <pid>/fdinfo/<fd>
>
> So gputop could just read this file to know where the info is.
> Too bad we can't symlink cross fs, otherwise we could just add symlinks
> to e.g. /sys/class/drm/card<N>/clients/*, which then nicely separate it
> per gpu too.
>
> But see below.
>
> >
> >>I did some research into sysfs binary attributes, and while some sources mention that
> >>it's often used for dumping or loading of driver FW, none of them claim it cannot
> >>be used for other purposes.
> >>
> >>>>>---
> >>>>>   drivers/gpu/drm/drm_internal.h       |  2 +-
> >>>>>   drivers/gpu/drm/drm_privacy_screen.c |  2 +-
> >>>>>   drivers/gpu/drm/drm_sysfs.c          | 89 ++++++++++++++++++++++------
> >>>>>   3 files changed, 74 insertions(+), 19 deletions(-)
> >>>>>
> >>>>>diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
> >>>>>index 2215baef9a3e..9a399b03d11c 100644
> >>>>>--- a/drivers/gpu/drm/drm_internal.h
> >>>>>+++ b/drivers/gpu/drm/drm_internal.h
> >>>>>@@ -145,7 +145,7 @@ bool drm_master_internal_acquire(struct drm_device *dev);
> >>>>>   void drm_master_internal_release(struct drm_device *dev);
> >>>>>
> >>>>>   /* drm_sysfs.c */
> >>>>>-extern struct class *drm_class;
> >>>>>+extern struct class drm_class;
> >>>>>
> >>>>>   int drm_sysfs_init(void);
> >>>>>   void drm_sysfs_destroy(void);
> >>>>>diff --git a/drivers/gpu/drm/drm_privacy_screen.c b/drivers/gpu/drm/drm_privacy_screen.c
> >>>>>index 6cc39e30781f..2fbd24ba5818 100644
> >>>>>--- a/drivers/gpu/drm/drm_privacy_screen.c
> >>>>>+++ b/drivers/gpu/drm/drm_privacy_screen.c
> >>>>>@@ -401,7 +401,7 @@ struct drm_privacy_screen *drm_privacy_screen_register(
> >>>>>          mutex_init(&priv->lock);
> >>>>>          BLOCKING_INIT_NOTIFIER_HEAD(&priv->notifier_head);
> >>>>>
> >>>>>-       priv->dev.class = drm_class;
> >>>>>+       priv->dev.class = &drm_class;
> >>>>>          priv->dev.type = &drm_privacy_screen_type;
> >>>>>          priv->dev.parent = parent;
> >>>>>          priv->dev.release = drm_privacy_screen_device_release;
> >>>>>diff --git a/drivers/gpu/drm/drm_sysfs.c b/drivers/gpu/drm/drm_sysfsc
> >>>>>index a953f69a34b6..56ca9e22c720 100644
> >>>>>--- a/drivers/gpu/drm/drm_sysfs.c
> >>>>>+++ b/drivers/gpu/drm/drm_sysfs.c
> >>>>>@@ -58,8 +58,6 @@ static struct device_type drm_sysfs_device_connector = {
> >>>>>          .name = "drm_connector",
> >>>>>   };
> >>>>>
> >>>>>-struct class *drm_class;
> >>>>>-
> >>>>>   #ifdef CONFIG_ACPI
> >>>>>   static bool drm_connector_acpi_bus_match(struct device *dev)
> >>>>>   {
> >>>>>@@ -128,6 +126,62 @@ static const struct component_ops typec_connector_ops = {
> >>>>>
> >>>>>   static CLASS_ATTR_STRING(version, S_IRUGO, "drm 1.1.0 20060810");
> >>>>>
> >>>>>+static ssize_t clients_show(struct device *cd, struct device_attribute *attr, char *buf)
> >>>>>+{
> >>>>>+       struct drm_minor *minor = cd->driver_data;
> >>>>>+       struct drm_device *ddev = minor->dev;
> >>>>>+       struct drm_file *priv;
> >>>>>+       ssize_t offset = 0;
> >>>>>+       void *pid_buf;
> >>>>>+
> >>>>>+       if (minor->type != DRM_MINOR_RENDER)
> >>>>>+               return 0;
> >>>
> >>>Why this?
> >>
> >>I return nothing in case of a non-render node because we don't want display drivers
> >>to confuse UM GPU profilers.
> >
> >Feels to arbitrary to me. Let them handle it.
> >
> >>>>>+
> >>>>>+       pid_buf = kvmalloc(PAGE_SIZE, GFP_KERNEL);
> >>>
> >>>I don't quite get the kvmalloc for just one page (or why even a temporay
> >>>buffer and not write into buf directly?).
> >>
> >>Should've used kmalloc, you're right. Or else I could just write everything straight into 'buf'.
> >>
> >>>>>+       if (!pid_buf)
> >>>>>+               return 0;
> >>>>>+
> >>>>>+       mutex_lock(&ddev->filelist_mutex);
> >>>>>+       list_for_each_entry_reverse(priv, &ddev->filelist, lhead) {
> >>>>>+               struct pid *pid;
> >>>>>+
> >>>>>+               if (drm_WARN_ON(ddev, (PAGE_SIZE - offset) < sizeof(pid_t)))
> >>>>>+                       break;
> >>>
> >>>Feels bad.. I would suggest exploring implementing a read callback (instead of
> >>>show) and handling arbitrary size output.
> >>
> >>I think regular class attributes can only implement show() and set(). For a more complex
> >>interface, I would have to turn it into an actual binary attribute, and that would be the only
> >>choice if we want the list of clients to be of arbitrary size.
> >
> >Yeah, i915 uses that to dump the error capture file which can be huge
> >and is text so it is doable.
> >
> >>>>>+
> >>>>>+               rcu_read_lock();
> >>>>>+               pid = rcu_dereference(priv->pid);
> >>>>>+               (*(pid_t *)(pid_buf + offset)) = pid_vnr(pid);
> >>>>>+               rcu_read_unlock();
> >>>>>+
> >>>>>+               offset += sizeof(pid_t);
> >>>>>+       }
> >>>>>+       mutex_unlock(&ddev->filelist_mutex);
> >>>>>+
> >>>>>+       if (offset < PAGE_SIZE)
> >>>>>+               (*(pid_t *)(pid_buf + offset)) = 0;
> >>>
> >>>Either NULL terminated or PAGE_SIZE/sizeof(pid) entries and not NULL
> >>>terminated feels weird. If I got that right.
> >>>
> >>>For me everything points towards going for text output.
> >>
> >>Yes, I know it might sound weird, but my reasoning was: either there are PAGE_SIZE/sizeof(pid) entries
> >>and the file isn't NULL terminated (which should be picked up by clients as being one page worth
> >>of data, the sysfs attribute maximum transfer unit), or else there aren't enough entries to fill
> >>a page and after the last one there's a NULL entry.
> >>
> >>
> >>>>>+
> >>>>>+       memcpy(buf, pid_buf, offset);
> >>>>>+
> >>>>>+       kvfree(pid_buf);
> >>>>>+
> >>>>>+       return offset;
> >>>>>+
> >>>>>+}
> >>>>>+static DEVICE_ATTR_RO(clients);
>
>
> /proc/<pid>/fdinfo/ is only readable by the owner. if we report what are
> the open fds (or even what are the pids with a drm fd), we are
> leaking that info. So we should probably make this
> DEVICE_ATTR_ADMIN_RO.

I think this is an argument for _only_ listing the PID and not full
paths.  I have a use-case where I'd prefer it not to be
DEVICE_ATTR_ADMIN_RO

That said, I think I do prefer txt rather than binary.  If nothing
else, it makes it easier to deal with for scripting, which is also
useful sometimes.

BR,
-R

> Lucas De Marchi
>
> >>>
> >>>Shouldn't BIN_ATTR_RO be used for binary files in sysfs?
> >>
> >>Like I said above, I sort of faked a binary attribute through the regular sysfs attr API,
> >>which is most likely a bad idea.
> >>
> >>>Regards,
> >>>
> >>>Tvrtko
> >>>
> >>>P.S. Or maybe it is time for drmfs? Where each client gets a directory and
> >>>drivers can populate files. Such as per client logging streams and whatnot.
> >>
> >>Yes, but maybe this is something we can discuss in depth in an RFC at a later time?
> >
> >Yes of course, it is just a long standing idea for flexible per client
> >stuff.
> >
> >Regards,
> >
> >Tvrtko
> >
> >>
> >>>>>+
> >>>>>+static struct attribute *drm_device_attrs[] = {
> >>>>>+       &dev_attr_clients.attr,
> >>>>>+       NULL,
> >>>>>+};
> >>>>>+ATTRIBUTE_GROUPS(drm_device);
> >>>>>+
> >>>>>+struct class drm_class = {
> >>>>>+       .name           = "drm",
> >>>>>+       .dev_groups     = drm_device_groups,
> >>>>>+};
> >>>>>+
> >>>>>+static bool drm_class_initialised;
> >>>>>+
> >>>>>   /**
> >>>>>    * drm_sysfs_init - initialize sysfs helpers
> >>>>>    *
> >>>>>@@ -142,18 +196,19 @@ int drm_sysfs_init(void)
> >>>>>   {
> >>>>>          int err;
> >>>>>
> >>>>>-       drm_class = class_create("drm");
> >>>>>-       if (IS_ERR(drm_class))
> >>>>>-               return PTR_ERR(drm_class);
> >>>>>+       err = class_register(&drm_class);
> >>>>>+       if (err)
> >>>>>+               return err;
> >>>>>
> >>>>>-       err = class_create_file(drm_class, &class_attr_version.attr);
> >>>>>+       err = class_create_file(&drm_class, &class_attr_version.attr);
> >>>>>          if (err) {
> >>>>>-               class_destroy(drm_class);
> >>>>>-               drm_class = NULL;
> >>>>>+               class_destroy(&drm_class);
> >>>>>                  return err;
> >>>>>          }
> >>>>>
> >>>>>-       drm_class->devnode = drm_devnode;
> >>>>>+       drm_class.devnode = drm_devnode;
> >>>>>+
> >>>>>+       drm_class_initialised = true;
> >>>>>
> >>>>>          drm_sysfs_acpi_register();
> >>>>>          return 0;
> >>>>>@@ -166,12 +221,12 @@ int drm_sysfs_init(void)
> >>>>>    */
> >>>>>   void drm_sysfs_destroy(void)
> >>>>>   {
> >>>>>-       if (IS_ERR_OR_NULL(drm_class))
> >>>>>+       if (!drm_class_initialised)
> >>>>>                  return;
> >>>>>          drm_sysfs_acpi_unregister();
> >>>>>-       class_remove_file(drm_class, &class_attr_version.attr);
> >>>>>-       class_destroy(drm_class);
> >>>>>-       drm_class = NULL;
> >>>>>+       class_remove_file(&drm_class, &class_attr_version.attr);
> >>>>>+       class_destroy(&drm_class);
> >>>>>+       drm_class_initialised = false;
> >>>>>   }
> >>>>>
> >>>>>   static void drm_sysfs_release(struct device *dev)
> >>>>>@@ -372,7 +427,7 @@ int drm_sysfs_connector_add(struct drm_connector *connector)
> >>>>>                  return -ENOMEM;
> >>>>>
> >>>>>          device_initialize(kdev);
> >>>>>-       kdev->class = drm_class;
> >>>>>+       kdev->class = &drm_class;
> >>>>>          kdev->type = &drm_sysfs_device_connector;
> >>>>>          kdev->parent = dev->primary->kdev;
> >>>>>          kdev->groups = connector_dev_groups;
> >>>>>@@ -550,7 +605,7 @@ struct device *drm_sysfs_minor_alloc(struct drm_minor *minor)
> >>>>>                          minor_str = "card%d";
> >>>>>
> >>>>>                  kdev->devt = MKDEV(DRM_MAJOR, minor->index);
> >>>>>-               kdev->class = drm_class;
> >>>>>+               kdev->class = &drm_class;
> >>>>>                  kdev->type = &drm_sysfs_device_minor;
> >>>>>          }
> >>>>>
> >>>>>@@ -579,10 +634,10 @@ struct device *drm_sysfs_minor_alloc(struct drm_minor *minor)
> >>>>>    */
> >>>>>   int drm_class_device_register(struct device *dev)
> >>>>>   {
> >>>>>-       if (!drm_class || IS_ERR(drm_class))
> >>>>>+       if (!drm_class_initialised)
> >>>>>                  return -ENOENT;
> >>>>>
> >>>>>-       dev->class = drm_class;
> >>>>>+       dev->class = &drm_class;
> >>>>>          return device_register(dev);
> >>>>>   }
> >>>>>   EXPORT_SYMBOL_GPL(drm_class_device_register);
> >>>>>
> >>>>>base-commit: 45c734fdd43db14444025910b4c59dd2b8be714a
> >>>>>--
> >>>>>2.44.0
> >>>>>
> >>
> >>Adrian Larumbe

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