Re: sched/debug: CPU hotplug operation suffers in a large cpu systems

[Michael Ellerman <mpe@ellerman.id.au>]

Phil Auld <pauld@redhat.com> writes:
> On Tue, Dec 13, 2022 at 07:23:54AM +0100 Greg Kroah-Hartman wrote:
>> On Mon, Dec 12, 2022 at 02:17:58PM -0500, Phil Auld wrote:
>> > Hi,
>> > 
>> > On Tue, Nov 08, 2022 at 01:24:39PM +0100 Greg Kroah-Hartman wrote:
>> > > On Tue, Nov 08, 2022 at 03:30:46PM +0530, Vishal Chourasia wrote:
>> > > > 
>> > > > Thanks Greg & Peter for your direction. 
>> > > > 
>> > > > While we pursue the idea of having debugfs based on kernfs, we thought about
>> > > > having a boot time parameter which would disable creating and updating of the
>> > > > sched_domain debugfs files and this would also be useful even when the kernfs
>> > > > solution kicks in, as users who may not care about these debugfs files would
>> > > > benefit from a faster CPU hotplug operation.
>> > > 
>> > > Ick, no, you would be adding a new user/kernel api that you will be
>> > > required to support for the next 20+ years.  Just to get over a
>> > > short-term issue before you solve the problem properly.
>> > 
>> > I'm not convinced moving these files from debugfs to kernfs is the right
>> > fix.  That will take it from ~50 back to ~20 _minutes_ on these systems.
>> > I don't think either of those numbers is reasonable.
>> > 
>> > The issue as I see it is the full rebuild for every change with no way to
>> > batch the changes. How about something like the below?
>> > 
>> > This puts the domains/* files under the sched_verbose flag. About the only
>> > thing under that flag now are the detailed topology discovery printks anyway
>> > so this fits together nicely.
>> > 
>> > This way the files would be off by default (assuming you don't boot with
>> > sched_verbose) and can be created at runtime by enabling verbose. Multiple
>> > changes could also be batched by disabling/makeing changes/re-enabling.
>> > 
>> > It does not create a new API, uses one that is already there.
>> 
>> The idea seems good, the implementation might need a bit of work :)
>
> More than the one comment below? Let me know.
>
>> 
>> > > If you really do not want these debugfs files, just disable debugfs from
>> > > your system.  That should be a better short-term solution, right?
>> > 
>> > We do find these files useful at times for debugging issue and looking
>> > at what's going on on the system.
>> > 
>> > > 
>> > > Or better yet, disable SCHED_DEBUG, why can't you do that?
>> > 
>> > Same with this... useful information with (modulo issues like this)
>> > small cost. There are also tuning knobs that are only available
>> > with SCHED_DEBUG. 
>> > 
>> > 
>> > Cheers,
>> > Phil
>> > 
>> > ---------------
>> > 
>> > sched/debug: Put sched/domains files under verbose flag
>> > 
>> > The debug files under sched/domains can take a long time to regenerate,
>> > especially when updates are done one at a time. Move these files under
>> > the verbose debug flag. Allow changes to verbose to trigger generation
>> > of the files. This lets a user batch the updates but still have the
>> > information available.  The detailed topology printk messages are also
>> > under verbose.
>> > 
>> > Signed-off-by: Phil Auld <pauld@redhat.com>
>> > ---
>> >  kernel/sched/debug.c | 68 ++++++++++++++++++++++++++++++++++++++++++--
>> >  1 file changed, 66 insertions(+), 2 deletions(-)
>> > 
>> > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
>> > index 1637b65ba07a..2eb51ee3ccab 100644
>> > --- a/kernel/sched/debug.c
>> > +++ b/kernel/sched/debug.c
>> > @@ -280,6 +280,31 @@ static const struct file_operations sched_dynamic_fops = {
>> >  
>> >  __read_mostly bool sched_debug_verbose;
>> >  
>> > +static ssize_t sched_verbose_write(struct file *filp, const char __user *ubuf,
>> > +				   size_t cnt, loff_t *ppos);
>> > +
>> > +static int sched_verbose_show(struct seq_file *m, void *v)
>> > +{
>> > +	if (sched_debug_verbose)
>> > +		seq_puts(m,"Y\n");
>> > +	else
>> > +		seq_puts(m,"N\n");
>> > +	return 0;
>> > +}
>> > +
>> > +static int sched_verbose_open(struct inode *inode, struct file *filp)
>> > +{
>> > +	return single_open(filp, sched_verbose_show, NULL);
>> > +}
>> > +
>> > +static const struct file_operations sched_verbose_fops = {
>> > +	.open		= sched_verbose_open,
>> > +	.write		= sched_verbose_write,
>> > +	.read		= seq_read,
>> > +	.llseek		= seq_lseek,
>> > +	.release	= seq_release,
>> > +};
>> > +
>> >  static const struct seq_operations sched_debug_sops;
>> >  
>> >  static int sched_debug_open(struct inode *inode, struct file *filp)
>> > @@ -303,7 +328,7 @@ static __init int sched_init_debug(void)
>> >  	debugfs_sched = debugfs_create_dir("sched", NULL);
>> >  
>> >  	debugfs_create_file("features", 0644, debugfs_sched, NULL, &sched_feat_fops);
>> > -	debugfs_create_bool("verbose", 0644, debugfs_sched, &sched_debug_verbose);
>> > +	debugfs_create_file("verbose", 0644, debugfs_sched, NULL, &sched_verbose_fops);
>> >  #ifdef CONFIG_PREEMPT_DYNAMIC
>> >  	debugfs_create_file("preempt", 0644, debugfs_sched, NULL, &sched_dynamic_fops);
>> >  #endif
>> > @@ -402,15 +427,23 @@ void update_sched_domain_debugfs(void)
>> >  	if (!debugfs_sched)
>> >  		return;
>> >  
>> > +	if (!sched_debug_verbose)
>> > +		return;
>> > +
>> >  	if (!cpumask_available(sd_sysctl_cpus)) {
>> >  		if (!alloc_cpumask_var(&sd_sysctl_cpus, GFP_KERNEL))
>> >  			return;
>> >  		cpumask_copy(sd_sysctl_cpus, cpu_possible_mask);
>> >  	}
>> >  
>> > -	if (!sd_dentry)
>> > +	if (!sd_dentry) {
>> >  		sd_dentry = debugfs_create_dir("domains", debugfs_sched);
>> >  
>> > +		/* rebuild sd_sysclt_cpus if empty since it gets cleared below */
>> > +		if (cpumask_first(sd_sysctl_cpus) >=  nr_cpu_ids)
>> > +			cpumask_copy(sd_sysctl_cpus, cpu_online_mask);
>> > +	}
>> > +
>> >  	for_each_cpu(cpu, sd_sysctl_cpus) {
>> >  		struct sched_domain *sd;
>> >  		struct dentry *d_cpu;
>> > @@ -443,6 +476,37 @@ void dirty_sched_domain_sysctl(int cpu)
>> >  
>> >  #endif /* CONFIG_SMP */
>> >  
>> > +static ssize_t sched_verbose_write(struct file *filp, const char __user *ubuf,
>> > +				   size_t cnt, loff_t *ppos)
>> > +{
>> > +	struct dentry *dentry = filp->f_path.dentry;
>> > +	bool orig = sched_debug_verbose;
>> > +	bool bv;
>> > +	int r;
>> > +
>> > +	r = kstrtobool_from_user(ubuf, cnt, &bv);
>> > +	if (!r) {
>> > +		mutex_lock(&sched_domains_mutex);
>> > +		r = debugfs_file_get(dentry);
>> > +		if (unlikely(r))
>> > +			return r;
>> > +		sched_debug_verbose = bv;
>> > +		debugfs_file_put(dentry);
>> 
>> Why the get/put of the debugfs dentry? for just this single value?
>
> That's what debugfs_file_write_bool() does, which is where I got that since
> that's really what this is doing. I couldn't see a good way to make this
> just call that.

I think you can do it like below? Only lightly tested :)

cheers


diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 1637b65ba07a..bc96380cf336 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -280,6 +279,42 @@ static const struct file_operations sched_dynamic_fops = {
 
 __read_mostly bool sched_debug_verbose;
 
+#ifdef CONFIG_SMP
+static struct dentry		*sd_dentry;
+
+static ssize_t sched_verbose_write(struct file *filp, const char __user *ubuf,
+				   size_t cnt, loff_t *ppos)
+{
+	ssize_t result;
+	bool orig;
+
+	mutex_lock(&sched_domains_mutex);
+
+	orig = sched_debug_verbose;
+	result = debugfs_write_file_bool(filp, ubuf, cnt, ppos);
+
+	if (sched_debug_verbose && !orig)
+		update_sched_domain_debugfs();
+	else if (!sched_debug_verbose && orig) {
+		debugfs_remove(sd_dentry);
+		sd_dentry = NULL;
+	}
+
+	mutex_unlock(&sched_domains_mutex);
+
+	return result;
+}
+#else
+#define sched_verbose_write debugfs_write_file_bool
+#endif
+
+static const struct file_operations sched_verbose_fops = {
+	.read =		debugfs_read_file_bool,
+	.write =	sched_verbose_write,
+	.open =		simple_open,
+	.llseek =	default_llseek,
+};
+
 static const struct seq_operations sched_debug_sops;
 
 static int sched_debug_open(struct inode *inode, struct file *filp)
@@ -303,7 +338,7 @@ static __init int sched_init_debug(void)
 	debugfs_sched = debugfs_create_dir("sched", NULL);
 
 	debugfs_create_file("features", 0644, debugfs_sched, NULL, &sched_feat_fops);
-	debugfs_create_bool("verbose", 0644, debugfs_sched, &sched_debug_verbose);
+	debugfs_create_file_unsafe("verbose", 0644, debugfs_sched, &sched_debug_verbose, &sched_verbose_fops);
 #ifdef CONFIG_PREEMPT_DYNAMIC
 	debugfs_create_file("preempt", 0644, debugfs_sched, NULL, &sched_dynamic_fops);
 #endif
@@ -345,7 +380,6 @@ late_initcall(sched_init_debug);
 #ifdef CONFIG_SMP
 
 static cpumask_var_t		sd_sysctl_cpus;
-static struct dentry		*sd_dentry;
 
 static int sd_flags_show(struct seq_file *m, void *v)
 {
@@ -402,15 +436,23 @@ void update_sched_domain_debugfs(void)
 	if (!debugfs_sched)
 		return;
 
+	if (!sched_debug_verbose)
+		return;
+
 	if (!cpumask_available(sd_sysctl_cpus)) {
 		if (!alloc_cpumask_var(&sd_sysctl_cpus, GFP_KERNEL))
 			return;
 		cpumask_copy(sd_sysctl_cpus, cpu_possible_mask);
 	}
 
-	if (!sd_dentry)
+	if (!sd_dentry) {
 		sd_dentry = debugfs_create_dir("domains", debugfs_sched);
 
+		/* rebuild sd_sysclt_cpus if empty since it gets cleared below */
+		if (cpumask_first(sd_sysctl_cpus) >=  nr_cpu_ids)
+			cpumask_copy(sd_sysctl_cpus, cpu_online_mask);
+	}
+
 	for_each_cpu(cpu, sd_sysctl_cpus) {
 		struct sched_domain *sd;
 		struct dentry *d_cpu;