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trace/pid_list: optimize pid_list->lock contention 

When the system has many cores and task switching is frequent,
setting set_ftrace_pid can cause frequent pid_list->lock contention
and high system sys usage.

For example, in a 288-core VM environment, we observed 267 CPUs
experiencing contention on pid_list->lock, with stack traces showing:

 #4 [ffffa6226fb4bc70] native_queued_spin_lock_slowpath at ffffffff99cd4b7e
 #5 [ffffa6226fb4bc90] _raw_spin_lock_irqsave at ffffffff99cd3e36
 #6 [ffffa6226fb4bca0] trace_pid_list_is_set at ffffffff99267554
 #7 [ffffa6226fb4bcc0] trace_ignore_this_task at ffffffff9925c288
 #8 [ffffa6226fb4bcd8] ftrace_filter_pid_sched_switch_probe at ffffffff99246efe
 #9 [ffffa6226fb4bcf0] __schedule at ffffffff99ccd161

Replaces the existing spinlock with a seqlock to allow concurrent readers,
while maintaining write exclusivity.

Link: https://patch.msgid.link/20251113000252.1058144-1-leonylgao@gmail.com
Reviewed-by: Huang Cun <cunhuang@tencent.com>
Signed-off-by: Yongliang Gao <leonylgao@tencent.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
This commit is contained in:
Yongliang Gao 2025-11-13 08:02:52 +08:00 committed by Steven Rostedt (Google)
parent e29aa918a9
commit 97e047f44d
2 changed files with 22 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

30
kernel/trace/pid_list.c
View File

@ -3,6 +3,7 @@
* Copyright (C) 2021 VMware Inc, Steven Rostedt <rostedt@goodmis.org>
*/
#include <linux/spinlock.h>
#include <linux/seqlock.h>
#include <linux/irq_work.h>
#include <linux/slab.h>
#include "trace.h"
@ -126,7 +127,7 @@ bool trace_pid_list_is_set(struct trace_pid_list *pid_list, unsigned int pid)
{
union upper_chunk *upper_chunk;
union lower_chunk *lower_chunk;
unsigned long flags;
unsigned int seq;
unsigned int upper1;
unsigned int upper2;
unsigned int lower;
@ -138,14 +139,16 @@ bool trace_pid_list_is_set(struct trace_pid_list *pid_list, unsigned int pid)
if (pid_split(pid, &upper1, &upper2, &lower) < 0)
return false;
raw_spin_lock_irqsave(&pid_list->lock, flags);
upper_chunk = pid_list->upper[upper1];
if (upper_chunk) {
lower_chunk = upper_chunk->data[upper2];
if (lower_chunk)
ret = test_bit(lower, lower_chunk->data);
}
raw_spin_unlock_irqrestore(&pid_list->lock, flags);
do {
seq = read_seqcount_begin(&pid_list->seqcount);
ret = false;
upper_chunk = pid_list->upper[upper1];
if (upper_chunk) {
lower_chunk = upper_chunk->data[upper2];
if (lower_chunk)
ret = test_bit(lower, lower_chunk->data);
}
} while (read_seqcount_retry(&pid_list->seqcount, seq));
return ret;
}
@ -178,6 +181,7 @@ int trace_pid_list_set(struct trace_pid_list *pid_list, unsigned int pid)
return -EINVAL;
raw_spin_lock_irqsave(&pid_list->lock, flags);
write_seqcount_begin(&pid_list->seqcount);
upper_chunk = pid_list->upper[upper1];
if (!upper_chunk) {
upper_chunk = get_upper_chunk(pid_list);
@ -199,6 +203,7 @@ int trace_pid_list_set(struct trace_pid_list *pid_list, unsigned int pid)
set_bit(lower, lower_chunk->data);
ret = 0;
out:
write_seqcount_end(&pid_list->seqcount);
raw_spin_unlock_irqrestore(&pid_list->lock, flags);
return ret;
}
@ -230,6 +235,7 @@ int trace_pid_list_clear(struct trace_pid_list *pid_list, unsigned int pid)
return -EINVAL;
raw_spin_lock_irqsave(&pid_list->lock, flags);
write_seqcount_begin(&pid_list->seqcount);
upper_chunk = pid_list->upper[upper1];
if (!upper_chunk)
goto out;
@ -250,6 +256,7 @@ int trace_pid_list_clear(struct trace_pid_list *pid_list, unsigned int pid)
}
}
out:
write_seqcount_end(&pid_list->seqcount);
raw_spin_unlock_irqrestore(&pid_list->lock, flags);
return 0;
}
@ -340,8 +347,10 @@ static void pid_list_refill_irq(struct irq_work *iwork)
again:
raw_spin_lock(&pid_list->lock);
write_seqcount_begin(&pid_list->seqcount);
upper_count = CHUNK_ALLOC - pid_list->free_upper_chunks;
lower_count = CHUNK_ALLOC - pid_list->free_lower_chunks;
write_seqcount_end(&pid_list->seqcount);
raw_spin_unlock(&pid_list->lock);
if (upper_count <= 0 && lower_count <= 0)
@ -370,6 +379,7 @@ static void pid_list_refill_irq(struct irq_work *iwork)
}
raw_spin_lock(&pid_list->lock);
write_seqcount_begin(&pid_list->seqcount);
if (upper) {
*upper_next = pid_list->upper_list;
pid_list->upper_list = upper;
@ -380,6 +390,7 @@ static void pid_list_refill_irq(struct irq_work *iwork)
pid_list->lower_list = lower;
pid_list->free_lower_chunks += lcnt;
}
write_seqcount_end(&pid_list->seqcount);
raw_spin_unlock(&pid_list->lock);
/*
@ -419,6 +430,7 @@ struct trace_pid_list *trace_pid_list_alloc(void)
init_irq_work(&pid_list->refill_irqwork, pid_list_refill_irq);
raw_spin_lock_init(&pid_list->lock);
seqcount_raw_spinlock_init(&pid_list->seqcount, &pid_list->lock);
for (i = 0; i < CHUNK_ALLOC; i++) {
union upper_chunk *chunk;

1
kernel/trace/pid_list.h
View File

@ -76,6 +76,7 @@ union upper_chunk {
};
struct trace_pid_list {
seqcount_raw_spinlock_t seqcount;
raw_spinlock_t lock;
struct irq_work refill_irqwork;
union upper_chunk *upper[UPPER1_SIZE]; // 1 or 2K in size