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

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

On Thu, 30 Sep 2021, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Wed, Sep 29, 2021 at 01:32:41AM +0300, Jani Nikula 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
>> 
>> Cc: Daniel Vetter <daniel@ffwll.ch>
>> Cc: Dave Airlie <airlied@gmail.com>
>> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
>
> I wonder whether we shouldn't just enable this when lock debugging is
> enabled? Otherwise we need to make sure CI have this set or it's not very
> useful. Or at least a default y if CONFIG_DEBUG_WW_MUTEX_SLOWPATH or
> something like that.

Added the conditional default y, as well as depends on DEBUG_KERNEL.

>
> Either way: 
>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Thanks,
Jani.


>
>> ---
>>  drivers/gpu/drm/Kconfig            | 13 ++++++++
>>  drivers/gpu/drm/drm_modeset_lock.c | 49 ++++++++++++++++++++++++++++--
>>  include/drm/drm_modeset_lock.h     |  8 +++++
>>  3 files changed, 68 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
>> index b17e231ca6f7..7334975c788b 100644
>> --- a/drivers/gpu/drm/Kconfig
>> +++ b/drivers/gpu/drm/Kconfig
>> @@ -100,6 +100,19 @@ 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
>> +	select STACKDEPOT
>> +	depends on EXPERT
>> +	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)
>>  	 */
>> -- 
>> 2.30.2
>> 

-- 
Jani Nikula, Intel Open Source Graphics Center