Re: [Intel-gfx] [PATCH v2] drm/locking: add backtrace for locking contended locks without backoff

[Jani Nikula <jani.nikula@intel.com>]

On Fri, 01 Oct 2021, Jani Nikula <jani.nikula@intel.com> wrote:
> If drm_modeset_lock() returns -EDEADLK, the caller is supposed to drop
> all currently held locks using drm_modeset_backoff(). Failing to do so
> will result in warnings and backtraces on the paths trying to lock a
> contended lock. Add support for optionally printing the backtrace on the
> path that hit the deadlock and didn't gracefully handle the situation.
>
> For example, the patch [1] inadvertently dropped the return value check
> and error return on replacing calc_watermark_data() with
> intel_compute_global_watermarks(). The backtraces on the subsequent
> locking paths hitting WARN_ON(ctx->contended) were unhelpful, but adding
> the backtrace to the deadlock path produced this helpful printout:
>
> <7> [98.002465] drm_modeset_lock attempting to lock a contended lock without backoff:
>    drm_modeset_lock+0x107/0x130
>    drm_atomic_get_plane_state+0x76/0x150
>    skl_compute_wm+0x251d/0x2b20 [i915]
>    intel_atomic_check+0x1942/0x29e0 [i915]
>    drm_atomic_check_only+0x554/0x910
>    drm_atomic_nonblocking_commit+0xe/0x50
>    drm_mode_atomic_ioctl+0x8c2/0xab0
>    drm_ioctl_kernel+0xac/0x140
>
> Add new CONFIG_DRM_DEBUG_MODESET_LOCK to enable modeset lock debugging
> with stack depot and trace.
>
> [1] https://lore.kernel.org/r/20210924114741.15940-4-jani.nikula@intel.com
>
> v2:
> - default y if DEBUG_WW_MUTEX_SLOWPATH (Daniel)
> - depends on DEBUG_KERNEL
>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: Dave Airlie <airlied@gmail.com>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Pushed to drm-misc-next, thanks for the review.

BR,
Jani.

> ---
>  drivers/gpu/drm/Kconfig            | 15 +++++++++
>  drivers/gpu/drm/drm_modeset_lock.c | 49 ++++++++++++++++++++++++++++--
>  include/drm/drm_modeset_lock.h     |  8 +++++
>  3 files changed, 70 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index 2a926d0de423..a4c020a9a0eb 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -100,6 +100,21 @@ config DRM_DEBUG_DP_MST_TOPOLOGY_REFS
>            This has the potential to use a lot of memory and print some very
>            large kernel messages. If in doubt, say "N".
>  
> +config DRM_DEBUG_MODESET_LOCK
> +	bool "Enable backtrace history for lock contention"
> +	depends on STACKTRACE_SUPPORT
> +	depends on DEBUG_KERNEL
> +	depends on EXPERT
> +	select STACKDEPOT
> +	default y if DEBUG_WW_MUTEX_SLOWPATH
> +	help
> +	  Enable debug tracing of failures to gracefully handle drm modeset lock
> +	  contention. A history of each drm modeset lock path hitting -EDEADLK
> +	  will be saved until gracefully handled, and the backtrace will be
> +	  printed when attempting to lock a contended lock.
> +
> +	  If in doubt, say "N".
> +
>  config DRM_FBDEV_EMULATION
>  	bool "Enable legacy fbdev support for your modesetting driver"
>  	depends on DRM
> diff --git a/drivers/gpu/drm/drm_modeset_lock.c b/drivers/gpu/drm/drm_modeset_lock.c
> index bf8a6e823a15..4d32b61fa1fd 100644
> --- a/drivers/gpu/drm/drm_modeset_lock.c
> +++ b/drivers/gpu/drm/drm_modeset_lock.c
> @@ -25,6 +25,7 @@
>  #include <drm/drm_crtc.h>
>  #include <drm/drm_device.h>
>  #include <drm/drm_modeset_lock.h>
> +#include <drm/drm_print.h>
>  
>  /**
>   * DOC: kms locking
> @@ -77,6 +78,45 @@
>  
>  static DEFINE_WW_CLASS(crtc_ww_class);
>  
> +#if IS_ENABLED(CONFIG_DRM_DEBUG_MODESET_LOCK)
> +static noinline depot_stack_handle_t __stack_depot_save(void)
> +{
> +	unsigned long entries[8];
> +	unsigned int n;
> +
> +	n = stack_trace_save(entries, ARRAY_SIZE(entries), 1);
> +
> +	return stack_depot_save(entries, n, GFP_NOWAIT | __GFP_NOWARN);
> +}
> +
> +static void __stack_depot_print(depot_stack_handle_t stack_depot)
> +{
> +	struct drm_printer p = drm_debug_printer("drm_modeset_lock");
> +	unsigned long *entries;
> +	unsigned int nr_entries;
> +	char *buf;
> +
> +	buf = kmalloc(PAGE_SIZE, GFP_NOWAIT | __GFP_NOWARN);
> +	if (!buf)
> +		return;
> +
> +	nr_entries = stack_depot_fetch(stack_depot, &entries);
> +	stack_trace_snprint(buf, PAGE_SIZE, entries, nr_entries, 2);
> +
> +	drm_printf(&p, "attempting to lock a contended lock without backoff:\n%s", buf);
> +
> +	kfree(buf);
> +}
> +#else /* CONFIG_DRM_DEBUG_MODESET_LOCK */
> +static depot_stack_handle_t __stack_depot_save(void)
> +{
> +	return 0;
> +}
> +static void __stack_depot_print(depot_stack_handle_t stack_depot)
> +{
> +}
> +#endif /* CONFIG_DRM_DEBUG_MODESET_LOCK */
> +
>  /**
>   * drm_modeset_lock_all - take all modeset locks
>   * @dev: DRM device
> @@ -225,7 +265,9 @@ EXPORT_SYMBOL(drm_modeset_acquire_fini);
>   */
>  void drm_modeset_drop_locks(struct drm_modeset_acquire_ctx *ctx)
>  {
> -	WARN_ON(ctx->contended);
> +	if (WARN_ON(ctx->contended))
> +		__stack_depot_print(ctx->stack_depot);
> +
>  	while (!list_empty(&ctx->locked)) {
>  		struct drm_modeset_lock *lock;
>  
> @@ -243,7 +285,8 @@ static inline int modeset_lock(struct drm_modeset_lock *lock,
>  {
>  	int ret;
>  
> -	WARN_ON(ctx->contended);
> +	if (WARN_ON(ctx->contended))
> +		__stack_depot_print(ctx->stack_depot);
>  
>  	if (ctx->trylock_only) {
>  		lockdep_assert_held(&ctx->ww_ctx);
> @@ -274,6 +317,7 @@ static inline int modeset_lock(struct drm_modeset_lock *lock,
>  		ret = 0;
>  	} else if (ret == -EDEADLK) {
>  		ctx->contended = lock;
> +		ctx->stack_depot = __stack_depot_save();
>  	}
>  
>  	return ret;
> @@ -296,6 +340,7 @@ int drm_modeset_backoff(struct drm_modeset_acquire_ctx *ctx)
>  	struct drm_modeset_lock *contended = ctx->contended;
>  
>  	ctx->contended = NULL;
> +	ctx->stack_depot = 0;
>  
>  	if (WARN_ON(!contended))
>  		return 0;
> diff --git a/include/drm/drm_modeset_lock.h b/include/drm/drm_modeset_lock.h
> index aafd07388eb7..b84693fbd2b5 100644
> --- a/include/drm/drm_modeset_lock.h
> +++ b/include/drm/drm_modeset_lock.h
> @@ -24,6 +24,8 @@
>  #ifndef DRM_MODESET_LOCK_H_
>  #define DRM_MODESET_LOCK_H_
>  
> +#include <linux/types.h> /* stackdepot.h is not self-contained */
> +#include <linux/stackdepot.h>
>  #include <linux/ww_mutex.h>
>  
>  struct drm_modeset_lock;
> @@ -51,6 +53,12 @@ struct drm_modeset_acquire_ctx {
>  	 */
>  	struct drm_modeset_lock *contended;
>  
> +	/*
> +	 * Stack depot for debugging when a contended lock was not backed off
> +	 * from.
> +	 */
> +	depot_stack_handle_t stack_depot;
> +
>  	/*
>  	 * list of held locks (drm_modeset_lock)
>  	 */

-- 
Jani Nikula, Intel Open Source Graphics Center