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Date: Wed, 1 May 2024 11:46:50 -0500
From: Lucas De Marchi <lucas.demarchi@intel.com>
To: Rob Clark <robdclark@gmail.com>
CC: Tvrtko Ursulin <tursulin@ursulin.net>, =?utf-8?Q?Adri=C3=A1n?= Larumbe
	<adrian.larumbe@collabora.com>, Maarten Lankhorst
	<maarten.lankhorst@linux.intel.com>, Maxime Ripard <mripard@kernel.org>,
	Thomas Zimmermann <tzimmermann@suse.de>, David Airlie <airlied@gmail.com>,
	Daniel Vetter <daniel@ffwll.ch>, Hans de Goede <hdegoede@redhat.com>,
	<kernel@collabora.com>, Boris Brezillon <boris.brezillon@collabora.com>,
	Christopher Healy <healych@amazon.com>, <dri-devel@lists.freedesktop.org>,
	<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] drm/sysfs: Add drm class-wide attribute to get active
 device clients
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On Wed, May 01, 2024 at 09:25:01AM GMT, Rob Clark wrote:
>On Wed, May 1, 2024 at 9:19 AM Lucas De Marchi <lucas.demarchi@intel.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@collabora.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@collabora.com>
>> >>>>>Cc: Tvrtko Ursulin <tursulin@ursulin.net>
>> >>>>>Cc: Christopher Healy <healych@amazon.com>
>> >>>>>Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.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_sysfs.c
>> >>>>>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

maybe... today you still can't get the pids that have a file open, even if you
don't get the exact fd number. And it must have some good reasons because the
dir is S_IRUGO|S_IXUGO, yet kernel refuses to let me see what is inside.
This is interesting and possibly relevant to the discussion:

7bc3fa0172a4 ("procfs: allow reading fdinfo with PTRACE_MODE_READ")

>
>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.

agreed. Even if it's only pid, just make it text and the app will use it
as is to open the fdinfo path.

Lucas De Marchi

>
>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