Commit 97e047f4 authored by Yongliang Gao's avatar Yongliang Gao Committed by Steven Rostedt (Google)
Browse files

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: default avatarHuang Cun <cunhuang@tencent.com>
Signed-off-by: default avatarYongliang Gao <leonylgao@tencent.com>
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
parent e29aa918

kernel/trace/pid_list.c

+21 −9
Original line number Diff line number Diff line
@@ -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);

	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);

		}

	raw_spin_unlock_irqrestore(&pid_list->lock, flags);

	} 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;

kernel/trace/pid_list.h

+1 −0
Original line number Diff line number Diff line
@@ -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