linux-kernel - [PATCH v3] x86/alternatives: remove false sharing in poke_int3

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Message-ID: <20250325043316.874518-1-edumazet@google.com>
Date: Tue, 25 Mar 2025 04:33:16 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H . Peter Anvin" <hpa@...or.com>, 
	Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel <linux-kernel@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Masami Hiramatsu <mhiramat@...nel.org>, x86@...nel.org, 
	bpf@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>, 
	Greg Thelen <gthelen@...gle.com>, Stephane Eranian <eranian@...gle.com>, 
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH v3] x86/alternatives: remove false sharing in poke_int3_handler()

eBPF programs can be run 50,000,000 times per second on busy servers.

Whenever /proc/sys/kernel/bpf_stats_enabled is turned off,
hundreds of calls sites are patched from text_poke_bp_batch()
and we see a huge loss of performance due to false sharing
on bp_desc.refs lasting up to three seconds.

   51.30%  server_bin       [kernel.kallsyms]           [k] poke_int3_handler
            |
            |--46.45%--poke_int3_handler
            |          exc_int3
            |          asm_exc_int3
            |          |
            |          |--24.26%--cls_bpf_classify
            |          |          tcf_classify
            |          |          __dev_queue_xmit
            |          |          ip6_finish_output2
            |          |          ip6_output
            |          |          ip6_xmit
            |          |          inet6_csk_xmit
            |          |          __tcp_transmit_skb

Fix this by replacing bp_desc.refs with a per-cpu bp_refs.

Before the patch, on a host with 240 cores (480 threads):

sysctl -wq kernel.bpf_stats_enabled=0

text_poke_bp_batch(nr_entries=164) : Took 2655300 usec

bpftool prog | grep run_time_ns
...
105: sched_cls  name hn_egress  tag 699fc5eea64144e3  gpl run_time_ns
3009063719 run_cnt 82757845 : average cost is 36 nsec per call

After this patch:

sysctl -wq kernel.bpf_stats_enabled=0

text_poke_bp_batch(nr_entries=164) : Took 702 usec

$ bpftool prog | grep run_time_ns
...
105: sched_cls  name hn_egress  tag 699fc5eea64144e3  gpl run_time_ns
1928223019 run_cnt 67682728 : average cost is 28 nsec per call

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 arch/x86/kernel/alternative.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index c71b575bf229..5d364e990055 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -2137,28 +2137,29 @@ struct text_poke_loc {
 struct bp_patching_desc {
 	struct text_poke_loc *vec;
 	int nr_entries;
-	atomic_t refs;
 };
 
+static DEFINE_PER_CPU(atomic_t, bp_refs);
+
 static struct bp_patching_desc bp_desc;
 
 static __always_inline
 struct bp_patching_desc *try_get_desc(void)
 {
-	struct bp_patching_desc *desc = &bp_desc;
+	atomic_t *refs = this_cpu_ptr(&bp_refs);
 
-	if (!raw_atomic_inc_not_zero(&desc->refs))
+	if (!raw_atomic_inc_not_zero(refs))
 		return NULL;
 
-	return desc;
+	return &bp_desc;
 }
 
 static __always_inline void put_desc(void)
 {
-	struct bp_patching_desc *desc = &bp_desc;
+	atomic_t *refs = this_cpu_ptr(&bp_refs);
 
 	smp_mb__before_atomic();
-	raw_atomic_dec(&desc->refs);
+	raw_atomic_dec(refs);
 }
 
 static __always_inline void *text_poke_addr(struct text_poke_loc *tp)
@@ -2191,9 +2192,9 @@ noinstr int poke_int3_handler(struct pt_regs *regs)
 	 * Having observed our INT3 instruction, we now must observe
 	 * bp_desc with non-zero refcount:
 	 *
-	 *	bp_desc.refs = 1		INT3
-	 *	WMB				RMB
-	 *	write INT3			if (bp_desc.refs != 0)
+	 *	bp_refs = 1		INT3
+	 *	WMB			RMB
+	 *	write INT3		if (bp_refs != 0)
 	 */
 	smp_rmb();
 
@@ -2299,7 +2300,8 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
 	 * Corresponds to the implicit memory barrier in try_get_desc() to
 	 * ensure reading a non-zero refcount provides up to date bp_desc data.
 	 */
-	atomic_set_release(&bp_desc.refs, 1);
+	for_each_possible_cpu(i)
+		atomic_set_release(per_cpu_ptr(&bp_refs, i), 1);
 
 	/*
 	 * Function tracing can enable thousands of places that need to be
@@ -2413,8 +2415,12 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
 	/*
 	 * Remove and wait for refs to be zero.
 	 */
-	if (!atomic_dec_and_test(&bp_desc.refs))
-		atomic_cond_read_acquire(&bp_desc.refs, !VAL);
+	for_each_possible_cpu(i) {
+		atomic_t *refs = per_cpu_ptr(&bp_refs, i);
+
+		if (unlikely(!atomic_dec_and_test(refs)))
+			atomic_cond_read_acquire(refs, !VAL);
+	}
 }
 
 static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,
-- 
2.49.0.395.g12beb8f557-goog