Re: [PATCH] drm/locking: add backtrace for locking contended locks without backoff
On Thu, 30 Sep 2021, Daniel Vetter 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.nik...@intel.com
>>
>> Cc: Daniel Vetter
>> Cc: Dave Airlie
>> Signed-off-by: Jani Nikula
>
> 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
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
>> #include
>> #include
>> +#include
>>
>> /**
>> * 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, );
>> +stack_trace_snprint(buf, PAGE_SIZE, entries, nr_entries, 2);
>> +
>> +drm_printf(, "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
Re: [PATCH] drm/locking: add backtrace for locking contended locks without backoff
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.nik...@intel.com
>
> Cc: Daniel Vetter
> Cc: Dave Airlie
> Signed-off-by: Jani Nikula
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.
Either way:
Reviewed-by: Daniel Vetter
> ---
> 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
> #include
> #include
> +#include
>
> /**
> * 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, );
> + stack_trace_snprint(buf, PAGE_SIZE, entries, nr_entries, 2);
> +
> + drm_printf(, "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(>locked)) {
> struct drm_modeset_lock
[PATCH] drm/locking: add backtrace for locking contended locks without backoff
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.nik...@intel.com
Cc: Daniel Vetter
Cc: Dave Airlie
Signed-off-by: Jani Nikula
---
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
#include
#include
+#include
/**
* 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, );
+ stack_trace_snprint(buf, PAGE_SIZE, entries, nr_entries, 2);
+
+ drm_printf(, "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(>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(>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 =
