Re: [PATCH] alloc_tag: avoid mem alloc and iter reset when reading allocinfo

Suren Baghdasaryan
2025-05-07T23:43:11Z

On Wed, May 7, 2025 at 6:19 PM David Wang <00107082@163.com> wrote:
>
> Hi,
> Just want to share how I notice those memory allocation behaivors: the cumulative counters~!
>
> With cumulative counters, I can identify which module keeps alloc/free memory, by the ratio between
> cumulative calls and remaining calls, and maybe an optimization could be applied.
> Following is top16 I got on my system:
>
> +-----------------------------------------+-------+------------------+--------------------+
> | alloc | calls | cumulative calls | ratio |
> +-----------------------------------------+-------+------------------+--------------------+
> | fs/seq_file.c:584 | 2 | 18064825 | 9032412.5 |
> | fs/seq_file.c:38 | 5 | 18148288 | 3629657.6 |
> | fs/seq_file.c:63 | 15 | 18153271 | 1210218.0666666667 |
> | net/core/skbuff.c:577 | 9 | 10679975 | 1186663.888888889 |
> | net/core/skbuff.c:658 | 21 | 11013437 | 524449.380952381 |
> | fs/select.c:168 | 7 | 2831226 | 404460.85714285716 |
> | lib/alloc_tag.c:51 | 1 | 340649 | 340649.0 | <--- Here I started
> | kernel/signal.c:455 | 1 | 300730 | 300730.0 |
> | fs/notify/inotify/inotify_fsnotify.c:96 | 1 | 249831 | 249831.0 |
> | fs/ext4/dir.c:675 | 3 | 519734 | 173244.66666666666 |
> | drivers/usb/host/xhci.c:1555 | 4 | 126402 | 31600.5 |
> | fs/locks.c:275 | 36 | 986957 | 27415.472222222223 |
> | fs/proc/inode.c:502 | 3 | 63753 | 21251.0 |
> | fs/pipe.c:125 | 123 | 2143378 | 17425.837398373984 |
> | net/core/scm.c:84 | 3 | 43267 | 14422.333333333334 |
> | fs/kernel_read_file.c:80 | 2 | 26910 | 13455.0 |
> +-----------------------------------------+-------+------------------+--------------------+
>
> I think this is another "good" usage for cumulative counters: if a module just keeps alloc/free memory,
> maybe it is good to move the memory alloc/free to somewhere less frequent.
>
> In the case of this patch, a memory allocation for each read-calls, can be moved to opan-calls.
>
> If interested, I can re-send the patch for cumulative counters for further discussions.

Yeah, my issue with cumulative counters is that while they might be
useful for some analyses, most usecases would probably not benefit
from them while sharing the performance overhead. OTOH making it
optional with a separate CONFIG that affects the content of the
/proc/allocinfo seems like a bad idea to me. Userspace parsers now
would have to check not only the file version but also whether this
kernel config is enabled, or handle a possibility of an additional
column in the output. Does not seem like a good solution to me.

All that said, I'm open to suggestions if there is a way to
incorporate cumulative counters that would not tax all other usecases
that do not need them.

>
>
> FYI
> David