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/[email protected]
Reviewed-by: Huang Cun <[email protected]>
Signed-off-by: Yongliang Gao <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: Yongliang Gao

kernel/trace/pid_list.c

@@ -3,6 +3,7 @@
  * Copyright (C) 2021 VMware Inc, Steven Rostedt <[email protected]>
  */
 #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;

kernel/trace/pid_list.h

@@ -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