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

David Wang
2025-05-08T17:17:59Z

The impact of accumulated iterator rewinding could be observed via:

sudo strace -T -e read cat /proc/allocinfo > /dev/null

Tens of read should be observed and each read should become slower and slower.

At 2025-05-09 00:58:25, "David Wang" <00107082@163.com> wrote:
>
>At 2025-05-09 00:34:27, "Kent Overstreet" <kent.overstreet@linux.dev> wrote:
>>On Fri, May 09, 2025 at 12:24:56AM +0800, David Wang wrote:
>>> At 2025-05-08 21:33:50, "Kent Overstreet" <kent.overstreet@linux.dev> wrote:
>>> >The first question is - does it matter? If the optimization is just for
>>> >/proc/allocinfo, who's reading it at a high enough rate that we care?
>>> >
>>> >If it's only being used interactively, it doesn't matter. If it's being
>>> >read at a high rate by some sort of profiling program, we'd want to skip
>>> >the text interface entirely and add an ioctl to read the data out in a
>>> >binary format.
>>> ...^_^, Actually, I have been running tools parsing /proc/allocinfo every 5 seconds
>>> ,and feeding data to a prometheus server for a quite long while...
>>> 5 seconds seems not that frequent, but I also have all other proc files to read,
>>> I would like optimization for all the proc files......
>>>
>>> Ioctl or other binary interfaces are indeed more efficient, but most are
>>> not well documented, while most proc files are self-documented. If proc files
>>> are efficient enough, I think I would stay with proc files even with a binary
>>> interface alternate tens of fold faster.
>>
>>This would be a perfect place for a binary interface, you just want to
>>return an array of
>>
>>struct allocated_by_ip {
>> u64 ip;
>> u64 bytes;
>>};
>
>
>
>>
>>Printing it in text form requires symbol table lookup, what you're
>>optimizing is noise compared to that and vsnprintf().
>
>Oh, no, this optimization is mostly achieved by avoiding iter rewinding, I think
>I talk about the extra memory allocation "too much"....
>These lines of code:
>- while ((ct = codetag_next_ct(&priv->iter)) != NULL && node)
>- node--;
>have accumulated way too much.
>Think it this way, advancing iterator n times takes 1%, reasonable noise
>compared to symbol lookup and printf(). The problem is seq_file() would
>restart about 80 times to read out all content of /proc/allocinfo, accumulated
>to a total 40*n iterator advancement, hence 1% become 40*1%, noise become significant.
>
>My test result shows an improvement from 7ms to 4ms:
>
>Timings before:
> $ time cat /proc/allocinfo > /dev/null
>
> real 0m0.007s
> user 0m0.000s
> sys 0m0.007s
>read-syscalls get slower and slower:
> read(3, "allocinfo - version: 1.0\n# <"..., 131072) = 4085 <0.000062>
> ...
> read(3, " 0 0 drivers/gp"..., 131072) = 4046 <0.000135>
> read(3, " 0 0 sound/core"..., 131072) = 4021 <0.000150>
> ...
>
>and with the change:
> $ time cat /proc/allocinfo > /dev/null
>
> real 0m0.004s
> user 0m0.000s
> sys 0m0.003s
>