RE: [PATCH v9 1/5] cpumask: introduce cpumap_print_list/bitmask_to_buf to support large bitmask and list

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Greg KH [mailto:gregkh@xxxxxxxxxxxxxxxxxxx]
> Sent: Saturday, August 7, 2021 1:22 AM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> Cc: andriy.shevchenko@xxxxxxxxxxxxxxx; yury.norov@xxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx;
> dave.hansen@xxxxxxxxx; linux@xxxxxxxxxxxxxxxxxx; rafael@xxxxxxxxxx;
> rdunlap@xxxxxxxxxxxxx; agordeev@xxxxxxxxxxxxx; sbrivio@xxxxxxxxxx;
> jianpeng.ma@xxxxxxxxx; valentin.schneider@xxxxxxx; peterz@xxxxxxxxxxxxx;
> bristot@xxxxxxxxxx; guodong.xu@xxxxxxxxxx; tangchengchang
> <tangchengchang@xxxxxxxxxx>; Zengtao (B) <prime.zeng@xxxxxxxxxxxxx>;
> yangyicong <yangyicong@xxxxxxxxxx>; tim.c.chen@xxxxxxxxxxxxxxx; Linuxarm
> <linuxarm@xxxxxxxxxx>; tiantao (H) <tiantao6@xxxxxxxxxxxxx>; Jonathan Cameron
> <jonathan.cameron@xxxxxxxxxx>
> Subject: Re: [PATCH v9 1/5] cpumask: introduce
> cpumap_print_list/bitmask_to_buf to support large bitmask and list
> 
> On Fri, Aug 06, 2021 at 11:02:47PM +1200, Barry Song wrote:
> > From: Tian Tao <tiantao6@xxxxxxxxxxxxx>
> >
> > The existing cpumap_print_to_pagebuf() is used by cpu topology and other
> > drivers to export hexadecimal bitmask and decimal list to userspace by
> > sysfs ABI.
> >
> > Right now, those drivers are using a normal attribute for this kind of
> > ABIs. A normal attribute typically has show entry as below:
> >
> > static ssize_t example_dev_show(struct device *dev,
> >                 struct device_attribute *attr, char *buf)
> > {
> > 	...
> > 	return cpumap_print_to_pagebuf(true, buf, &pmu_mmdc->cpu);
> > }
> > show entry of attribute has no offset and count parameters and this
> > means the file is limited to one page only.
> >
> > cpumap_print_to_pagebuf() API works terribly well for this kind of
> > normal attribute with buf parameter and without offset, count:
> >
> > static inline ssize_t
> > cpumap_print_to_pagebuf(bool list, char *buf, const struct cpumask *mask)
> > {
> > 	return bitmap_print_to_pagebuf(list, buf, cpumask_bits(mask),
> > 				       nr_cpu_ids);
> > }
> >
> > The problem is once we have many cpus, we have a chance to make bitmask
> > or list more than one page. Especially for list, it could be as complex
> > as 0,3,5,7,9,...... We have no simple way to know it exact size.
> >
> > It turns out bin_attribute is a way to break this limit. bin_attribute
> > has show entry as below:
> > static ssize_t
> > example_bin_attribute_show(struct file *filp, struct kobject *kobj,
> >              struct bin_attribute *attr, char *buf,
> >              loff_t offset, size_t count)
> > {
> > 	...
> > }
> >
> > With the new offset and count parameters, this makes sysfs ABI be able
> > to support file size more than one page. For example, offset could be
> > >= 4096.
> >
> > This patch introduces cpumap_print_bitmask/list_to_buf() and their bitmap
> > infrastructure bitmap_print_bitmask/list_to_buf() so that those drivers
> > can move to bin_attribute to support large bitmask and list. At the same
> > time, we have to pass those corresponding parameters such as offset, count
> > from bin_attribute to this new API.
> >
> > Signed-off-by: Tian Tao <tiantao6@xxxxxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> > Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> > Cc: Stefano Brivio <sbrivio@xxxxxxxxxx>
> > Cc: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
> > Cc: "Ma, Jianpeng" <jianpeng.ma@xxxxxxxxx>
> > Cc: Yury Norov <yury.norov@xxxxxxxxx>
> > Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> > Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> > ---
> >  include/linux/bitmap.h  |   6 +++
> >  include/linux/cpumask.h |  38 +++++++++++++++
> >  lib/bitmap.c            | 103 ++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 147 insertions(+)
> >
> > diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> > index a36cfcec4e77..37f36dad18bd 100644
> > --- a/include/linux/bitmap.h
> > +++ b/include/linux/bitmap.h
> > @@ -227,6 +227,12 @@ unsigned int bitmap_ord_to_pos(const unsigned long
> *bitmap, unsigned int ord, un
> >  int bitmap_print_to_pagebuf(bool list, char *buf,
> >  				   const unsigned long *maskp, int nmaskbits);
> >
> > +extern int bitmap_print_bitmask_to_buf(char *buf, const unsigned long
> *maskp,
> > +				      int nmaskbits, loff_t off, size_t count);
> > +
> > +extern int bitmap_print_list_to_buf(char *buf, const unsigned long *maskp,
> > +				      int nmaskbits, loff_t off, size_t count);
> > +
> 
> Why are you adding bitmap_print_list_to_buf() when no one uses it in
> this patch series?
> 
> Did I miss it somewhere?

Yes. It is used in every patch except the last one from Yury
which is only extending comment.

drivers/base/topology.c:
+static ssize_t name##_list_read(struct file *file, struct kobject *kobj,	\
+				struct bin_attribute *attr, char *buf,		\
+				loff_t off, size_t count)			\
+{										\
+	struct device *dev = kobj_to_dev(kobj);					\
+										\
+	return cpumap_print_list_to_buf(buf, topology_##mask(dev->id),		\
+					off, count);				\
 }


drivers/base/node.c:
+static inline ssize_t cpulist_read(struct file *file, struct kobject *kobj,
+				   struct bin_attribute *attr, char *buf,
+				   loff_t off, size_t count)
 {
-	...
+	cpumask_and(mask, cpumask_of_node(node_dev->dev.id), cpu_online_mask);
+	n = cpumap_print_list_to_buf(buf, mask, off, count);
+	free_cpumask_var(mask);

> 
> thanks,
> 
> greg k-h

Thanks
Barry