Merge tag 'core-rseq-2025-11-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull rseq updates from Thomas Gleixner:
 "A large overhaul of the restartable sequences and CID management:

  The recent enablement of RSEQ in glibc resulted in regressions which
  are caused by the related overhead. It turned out that the decision to
  invoke the exit to user work was not really a decision. More or less
  each context switch caused that. There is a long list of small issues
  which sums up nicely and results in a 3-4% regression in I/O
  benchmarks.

  The other detail which caused issues due to extra work in context
  switch and task migration is the CID (memory context ID) management.
  It also requires to use a task work to consolidate the CID space,
  which is executed in the context of an arbitrary task and results in
  sporadic uncontrolled exit latencies.

  The rewrite addresses this by:

   - Removing deprecated and long unsupported functionality

   - Moving the related data into dedicated data structures which are
     optimized for fast path processing.

   - Caching values so actual decisions can be made

   - Replacing the current implementation with a optimized inlined
     variant.

   - Separating fast and slow path for architectures which use the
     generic entry code, so that only fault and error handling goes into
     the TIF_NOTIFY_RESUME handler.

   - Rewriting the CID management so that it becomes mostly invisible in
     the context switch path. That moves the work of switching modes
     into the fork/exit path, which is a reasonable tradeoff. That work
     is only required when a process creates more threads than the
     cpuset it is allowed to run on or when enough threads exit after
     that. An artificial thread pool benchmarks which triggers this did
     not degrade, it actually improved significantly.

     The main effect in migration heavy scenarios is that runqueue lock
     held time and therefore contention goes down significantly"

* tag 'core-rseq-2025-11-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (54 commits)
  sched/mmcid: Switch over to the new mechanism
  sched/mmcid: Implement deferred mode change
  irqwork: Move data struct to a types header
  sched/mmcid: Provide CID ownership mode fixup functions
  sched/mmcid: Provide new scheduler CID mechanism
  sched/mmcid: Introduce per task/CPU ownership infrastructure
  sched/mmcid: Serialize sched_mm_cid_fork()/exit() with a mutex
  sched/mmcid: Provide precomputed maximal value
  sched/mmcid: Move initialization out of line
  signal: Move MMCID exit out of sighand lock
  sched/mmcid: Convert mm CID mask to a bitmap
  cpumask: Cache num_possible_cpus()
  sched/mmcid: Use cpumask_weighted_or()
  cpumask: Introduce cpumask_weighted_or()
  sched/mmcid: Prevent pointless work in mm_update_cpus_allowed()
  sched/mmcid: Move scheduler code out of global header
  sched: Fixup whitespace damage
  sched/mmcid: Cacheline align MM CID storage
  sched/mmcid: Use proper data structures
  sched/mmcid: Revert the complex CID management
  ...

2 files changed, 30 insertions, 1 deletions

diff --git a/init/Kconfig b/init/Kconfig
index cab3ad28ca49..d1c606ec632e 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1913,10 +1913,36 @@ config RSEQ
 
 	  If unsure, say Y.
 
+config RSEQ_STATS
+	default n
+	bool "Enable lightweight statistics of restartable sequences" if EXPERT
+	depends on RSEQ && DEBUG_FS
+	help
+	  Enable lightweight counters which expose information about the
+	  frequency of RSEQ operations via debugfs. Mostly interesting for
+	  kernel debugging or performance analysis. While lightweight it's
+	  still adding code into the user/kernel mode transitions.
+
+	  If unsure, say N.
+
+config RSEQ_DEBUG_DEFAULT_ENABLE
+	default n
+	bool "Enable restartable sequences debug mode by default" if EXPERT
+	depends on RSEQ
+	help
+	  This enables the static branch for debug mode of restartable
+	  sequences.
+
+	  This also can be controlled on the kernel command line via the
+	  command line parameter "rseq_debug=0/1" and through debugfs.
+
+	  If unsure, say N.
+
 config DEBUG_RSEQ
 	default n
 	bool "Enable debugging of rseq() system call" if EXPERT
-	depends on RSEQ && DEBUG_KERNEL
+	depends on RSEQ && DEBUG_KERNEL && !GENERIC_ENTRY
+	select RSEQ_DEBUG_DEFAULT_ENABLE
 	help
 	  Enable extra debugging checks for the rseq system call.
 
diff --git a/init/init_task.c b/init/init_task.c
index d970a847b657..49b13d7c3985 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -250,6 +250,9 @@ struct task_struct init_task __aligned(L1_CACHE_BYTES) = {
 #ifdef CONFIG_SECCOMP_FILTER
 	.seccomp	= { .filter_count = ATOMIC_INIT(0) },
 #endif
+#ifdef CONFIG_SCHED_MM_CID
+	.mm_cid		= { .cid = MM_CID_UNSET, },
+#endif
 };
 EXPORT_SYMBOL(init_task);