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

[Phil Auld <pauld@redhat.com>]

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 suppose the get/put may not be needed since the only way this should
go away is under that mutex too.

... erm, yeah, that return is a problem ... I'll fix that.

Also, this was originally on v6.1-rc7. I can rebase when I repost but I
didn't want to do it on a random commit so I picked (at the time) the latest
tag.  Should I just use the head of Linux? 

Thanks,
Phil

> 
> thanks,
> 
> greg k-h
> 

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