linux-kernel - [PATCH v13 11/14] unwind: Add USED bit to only have one conditional on way back to user space

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Message-ID: <20250708012359.516909531@kernel.org>
Date: Mon, 07 Jul 2025 21:22:50 -0400
From: Steven Rostedt <rostedt@...nel.org>
To: linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org,
 bpf@...r.kernel.org,
 x86@...nel.org
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...nel.org>,
 Jiri Olsa <jolsa@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>,
 Andrii Nakryiko <andrii@...nel.org>,
 Indu Bhagat <indu.bhagat@...cle.com>,
 "Jose E. Marchesi" <jemarch@....org>,
 Beau Belgrave <beaub@...ux.microsoft.com>,
 Jens Remus <jremus@...ux.ibm.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Jens Axboe <axboe@...nel.dk>,
 Florian Weimer <fweimer@...hat.com>,
 Sam James <sam@...too.org>
Subject: [PATCH v13 11/14] unwind: Add USED bit to only have one conditional on way back to user
 space

From: Steven Rostedt <rostedt@...dmis.org>

On the way back to user space, the function unwind_reset_info() is called
unconditionally (but always inlined). It currently has two conditionals.
One that checks the unwind_mask which is set whenever a deferred trace is
called and is used to know that the mask needs to be cleared. The other
checks if the cache has been allocated, and if so, it resets the
nr_entries so that the unwinder knows it needs to do the work to get a new
user space stack trace again (it only does it once per entering the
kernel).

Use one of the bits in the unwind mask as a "USED" bit that gets set
whenever a trace is created. This will make it possible to only check the
unwind_mask in the unwind_reset_info() to know if it needs to do work or
not and eliminates a conditional that happens every time the task goes
back to user space.

Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 include/linux/unwind_deferred.h | 14 +++++++-------
 kernel/unwind/deferred.c        |  5 ++++-
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/include/linux/unwind_deferred.h b/include/linux/unwind_deferred.h
index 587e120c0fd6..376bfd50ff75 100644
--- a/include/linux/unwind_deferred.h
+++ b/include/linux/unwind_deferred.h
@@ -21,6 +21,10 @@ struct unwind_work {
 #define UNWIND_PENDING_BIT	(BITS_PER_LONG - 1)
 #define UNWIND_PENDING		BIT(UNWIND_PENDING_BIT)

+/* Set if the unwinding was used (directly or deferred) */
+#define UNWIND_USED_BIT		(UNWIND_PENDING_BIT - 1)
+#define UNWIND_USED		BIT(UNWIND_USED_BIT)
+
 enum {
 	UNWIND_ALREADY_PENDING	= 1,
 	UNWIND_ALREADY_EXECUTED	= 2,
@@ -49,14 +53,10 @@ static __always_inline void unwind_reset_info(void)
 				return;
 		} while (!try_cmpxchg(&info->unwind_mask, &bits, 0UL));
 		current->unwind_info.id.id = 0;
+
+		if (unlikely(info->cache))
+			info->cache->nr_entries = 0;
 	}
-	/*
-	 * As unwind_user_faultable() can be called directly and
-	 * depends on nr_entries being cleared on exit to user,
-	 * this needs to be a separate conditional.
-	 */
-	if (unlikely(info->cache))
-		info->cache->nr_entries = 0;
 }

 #else /* !CONFIG_UNWIND_USER */
diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
index 256458f3eafe..9299974b6562 100644
--- a/kernel/unwind/deferred.c
+++ b/kernel/unwind/deferred.c
@@ -140,6 +140,9 @@ int unwind_user_faultable(struct unwind_stacktrace *trace)

 	cache->nr_entries = trace->nr;

+	/* Clear nr_entries on way back to user space */
+	set_bit(UNWIND_USED_BIT, &info->unwind_mask);
+
 	return 0;
 }

@@ -314,7 +317,7 @@ int unwind_deferred_init(struct unwind_work *work, unwind_callback_t func)
 	guard(mutex)(&callback_mutex);

 	/* See if there's a bit in the mask available */
-	if (unwind_mask == ~(UNWIND_PENDING))
+	if (unwind_mask == ~(UNWIND_PENDING|UNWIND_USED))
 		return -EBUSY;

 	work->bit = ffz(unwind_mask);
-- 
2.47.2