Messages in this thread | | | Date | Mon, 19 Jul 2021 10:10:16 -0700 | From | Yury Norov <> | Subject | Re: [PATCH v7 2/4] topology: use bin_attribute to break the size limitation of cpumap ABI |
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On Mon, Jul 19, 2021 at 11:10:45AM +0000, Song Bao Hua (Barry Song) wrote: > > > > -----Original Message----- > > From: andriy.shevchenko@linux.intel.com > > [mailto:andriy.shevchenko@linux.intel.com] > > Sent: Monday, July 19, 2021 9:07 PM > > To: Yury Norov <yury.norov@gmail.com> > > Cc: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>; > > gregkh@linuxfoundation.org; akpm@linux-foundation.org; > > linux-kernel@vger.kernel.org; dave.hansen@intel.com; > > linux@rasmusvillemoes.dk; rafael@kernel.org; rdunlap@infradead.org; > > agordeev@linux.ibm.com; sbrivio@redhat.com; jianpeng.ma@intel.com; > > valentin.schneider@arm.com; peterz@infradead.org; bristot@redhat.com; > > guodong.xu@linaro.org; tangchengchang <tangchengchang@huawei.com>; Zengtao (B) > > <prime.zeng@hisilicon.com>; yangyicong <yangyicong@huawei.com>; > > tim.c.chen@linux.intel.com; Linuxarm <linuxarm@huawei.com>; tiantao (H) > > <tiantao6@hisilicon.com> > > Subject: Re: [PATCH v7 2/4] topology: use bin_attribute to break the size > > limitation of cpumap ABI > > > > On Fri, Jul 16, 2021 at 06:12:21PM -0700, Yury Norov wrote: > > > On Sat, Jul 17, 2021 at 12:16:48AM +0000, Song Bao Hua (Barry Song) wrote: > > > > > From: Yury Norov [mailto:yury.norov@gmail.com] > > > > > Sent: Saturday, July 17, 2021 8:04 AM > > > > > On Fri, Jul 16, 2021 at 08:49:58AM +0000, Song Bao Hua (Barry Song) wrote: > > > > ... > > > > > > Generally good idea. However, for sysfs ABI entries, it might not be > > > > that true. > > > > > > > > A sysfs entry might never be read for its whole life. As I explained > > > > before, a sysfs entry - especially for list, is randomly "cat" by users. > > > > Many of them won't be read forever. And after they are read once, they > > > > will probably never be read again. The operations to read ABI could be > > > > random and rare. Performance wouldn't be a concern. > > > > > > > > To avoid holding the memory which might never be used, it is better to > > > > allocate and free the memory during runtime. I mean to allocate in show() > > > > and free in show(), aka, to do it on demand. > > > > > > > > For example, for a server with 256CPU and each cpu has dozens of sysfs ABI > > > > entries, only a few of sysfs list entries might be randomly "cat" by users. > > > > Holding 256*entries memory doesn't look good. > > > > > > Ok, makes sense. > > > > > > > > This would require to add bitmap_max_string_size(list, bitmap, nbits), > > > > > but it's O(1), and I think, others will find it helpful. > > > > > > > > What about getting size and memory at the same time? > > > > > > 1. We already have kasprintf() > > > 2. It breaks coding style. > > > > > > Documentation/process/coding-style.rst: > > > Functions should be short and sweet, and do just one thing. > > > > > > From practical point of view, there should be some balance between > > > granularity and ease-of-use. But in this case, bitmap_list cries for > > > a function that will help to estimate size of output buffer. > > > > According to the vsnprintf() logic the estimated size is what it returns. If > > user supplies too few bytes available, the comparison with the returned value > > can tell caller that space wasn't big enough.
snprintf(NULL, 0, "pbl", ...) also works, but it's O(nbits), and user is not guaranteed that allocated memory would be always sufficient. I mean max possible length for given nbits, not a length of a specific string.
In case of lists, the length may grow. Consider: 0-8 -> 0-3,5-8 -> 0,2,4,6,8
If we want to allocate a storage for strings that may change, it would be helpful to allocate memory for the lengthiest string in advance.
So, bitmap_max_string_len() may be a convenient O(1) alternative for those who interested in printing the same bitmap in the same buffer.
> As far as my understanding, for estimated size in bitmap_max_string_size() > Yury might mean something as below? > > * For bitmask: > Each 32bit needs 9 bytes "11111111,", so the max size of mask would be: > 9*nr_cpus / 32 ?
11111 -> "f1", but your formula gives 1. I think it should be like this (not tested): DIV_ROUND_UP(nbits, 4) + nbits < 32 ? 0 : nbits / 32 - 1
> * For list: > Maximally cpu0-9 need 2bytes, like "1," > Maximally cpu10-99 need 3bytes, like "50," > Maximally cpu100-999 need 4bytes, like "101," > Maximally cpu1000-9999 need 5 bytes.. > > So once we know the size of the bitmap, we can figure out the maximum > size of its string for mask and list? > > Pls correct me if you don't mean this, yury. Assuming that longest possible strings are of the form 0,2,4,6,... I think it's correct except for the last comma, so substract 1.
If we decide to go on with this bitmap_max_strlen(), the list part should be tested extensively. > > > And it's > > > easy to imagine a case where the estimated length of bitmap is needed > > > explicitly: > > > > > > size_t max_size = bitmap_max_string_size(nbits); > > > char *buf = kmalloc(PAGE_ALIGN(max_size) * nr_cpus); > > > > > > Thought, I don't insist. In your driver you can do: > > > > > > size_t size = snprintf(NULL, 0, ...); > > > void *buf = kmalloc(size); > > > > > > It will be fully correct, and you already have everything you need. > > > > > > > ssize_t bitmap_get_print_buf(bool list, char **buf, const unsigned long > > > > *maskp, int nmaskbits) > > > > > > > > ssize_t cpumap_get_print_buf(bool list, char **buf, const struct cpumask > > *mask); > > > > > > > > This API returns the size of printed buffer, and it also gets the > > > > printed result saved in *buf. Then drivers don't need to do three > > > > steps: > > > > > > > > 1. get cpumap buffer size which is your cpumap_max_string_size() > > > > 2. allocate memory for buffer according to size got in step 1 > > > > 3. print bitmap(cpumap) to buffer by "pbl" > > > > > > > > It will only need to call bitmap_get_print_buf() and all three > > > > things are done inside bitmap_get_print_buf(). > > > > > > > > How to use the size and memory allocated in cpumap_get_print_buf > > > > will be totally up to users. > > > > > > > > The other benefit for this is that if we get string size during initialization, > > > > and then we print in show() entries, the size got at the beginning might > > be not > > > > enough as system topology might have changed. Sysfs ABI reflects the status > > of > > > > system at this moment. > > > > -- > > With Best Regards, > > Andy Shevchenko > > > > Thanks > Barry
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