f2fs: sysfs: introduce critical_task_priority

This patch introduces /sys/fs/f2fs/<disk>/critical_task_priority, w/
this new sysfs interface, we can tune priority of f2fs_ckpt thread and
f2fs_gc thread.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: chaseyu

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -987,3 +987,11 @@ Contact:	"Chao Yu" <chao@kernel.org>
 Description:	f2fs can tune priority of thread which has entered into critical region covered by
 		f2fs rwsemphore lock. This sysfs entry can be used to control priority value, the
 		range is [100,139], by default the value is 120.
+
+What:		/sys/fs/f2fs/<disk>/critical_task_priority
+Date:		February 2026
+Contact:	"Chao Yu" <chao@kernel.org>
+Description:	It can be used to tune priority of f2fs critical task, e.g. f2fs_ckpt, f2fs_gc
+		threads, limitation as below:
+		- it requires user has CAP_SYS_NICE capability.
+		- the range is [100, 139], by default the value is 100.

fs/f2fs/checkpoint.c

@@ -2158,6 +2158,8 @@ int f2fs_start_ckpt_thread(struct f2fs_sb_info *sbi)
 	}
 
 	set_task_ioprio(cprc->f2fs_issue_ckpt, cprc->ckpt_thread_ioprio);
+	set_user_nice(cprc->f2fs_issue_ckpt,
+			PRIO_TO_NICE(sbi->critical_task_priority));
 
 	return 0;
 }

fs/f2fs/f2fs.h

@@ -1593,6 +1593,7 @@ enum node_type {
 #define MAX_LOCK_ELAPSED_TIME		500
 
 #define F2FS_DEFAULT_TASK_PRIORITY		(DEFAULT_PRIO)
+#define F2FS_CRITICAL_TASK_PRIORITY		NICE_TO_PRIO(0)
 
 static inline int f2fs_test_bit(unsigned int nr, char *addr);
 static inline void f2fs_set_bit(unsigned int nr, char *addr);
@@ -2010,6 +2011,9 @@ struct f2fs_sb_info {
 	/* adjust priority for task which is in critical region covered by lock */
 	unsigned int lock_duration_priority;
 
+	/* priority for critical task, e.g. f2fs_ckpt, f2fs_gc threads */
+	long critical_task_priority;
+
 #ifdef CONFIG_F2FS_FS_COMPRESSION
 	struct kmem_cache *page_array_slab;	/* page array entry */
 	unsigned int page_array_slab_size;	/* default page array slab size */

fs/f2fs/gc.c

@@ -234,6 +234,8 @@ int f2fs_start_gc_thread(struct f2fs_sb_info *sbi)
 		return err;
 	}
 
+	set_user_nice(gc_th->f2fs_gc_task,
+			PRIO_TO_NICE(sbi->critical_task_priority));
 	return 0;
 }
 

fs/f2fs/super.c

@@ -4340,6 +4340,7 @@ static void init_sb_info(struct f2fs_sb_info *sbi)
 	sbi->max_lock_elapsed_time = MAX_LOCK_ELAPSED_TIME;
 	sbi->adjust_lock_priority = 0;
 	sbi->lock_duration_priority = F2FS_DEFAULT_TASK_PRIORITY;
+	sbi->critical_task_priority = F2FS_CRITICAL_TASK_PRIORITY;
 
 	sbi->sum_blocksize = f2fs_sb_has_packed_ssa(sbi) ?
 		4096 : sbi->blocksize;

fs/f2fs/sysfs.c

@@ -969,6 +969,21 @@ static ssize_t __sbi_store(struct f2fs_attr *a,
 		return count;
 	}
 
+	if (!strcmp(a->attr.name, "critical_task_priority")) {
+		if (t < NICE_TO_PRIO(MIN_NICE) || t > NICE_TO_PRIO(MAX_NICE))
+			return -EINVAL;
+		if (!capable(CAP_SYS_NICE))
+			return -EPERM;
+		sbi->critical_task_priority = t;
+		if (sbi->cprc_info.f2fs_issue_ckpt)
+			set_user_nice(sbi->cprc_info.f2fs_issue_ckpt,
+					PRIO_TO_NICE(sbi->critical_task_priority));
+		if (sbi->gc_thread && sbi->gc_thread->f2fs_gc_task)
+			set_user_nice(sbi->gc_thread->f2fs_gc_task,
+					PRIO_TO_NICE(sbi->critical_task_priority));
+		return count;
+	}
+
 	__sbi_store_value(a, sbi, ptr + a->offset, t);
 
 	return count;
@@ -1288,6 +1303,7 @@ F2FS_SBI_GENERAL_RW_ATTR(bggc_io_aware);
 F2FS_SBI_GENERAL_RW_ATTR(max_lock_elapsed_time);
 F2FS_SBI_GENERAL_RW_ATTR(lock_duration_priority);
 F2FS_SBI_GENERAL_RW_ATTR(adjust_lock_priority);
+F2FS_SBI_GENERAL_RW_ATTR(critical_task_priority);
 
 /* STAT_INFO ATTR */
 #ifdef CONFIG_F2FS_STAT_FS
@@ -1496,6 +1512,7 @@ static struct attribute *f2fs_attrs[] = {
 	ATTR_LIST(max_lock_elapsed_time),
 	ATTR_LIST(lock_duration_priority),
 	ATTR_LIST(adjust_lock_priority),
+	ATTR_LIST(critical_task_priority),
 	NULL,
 };
 ATTRIBUTE_GROUPS(f2fs);