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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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 :)

> > 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?

thanks,

greg k-h