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

Kent Overstreet
2025-05-08T13:34:03Z

On Thu, May 08, 2025 at 01:51:48PM +0800, David Wang wrote:
> At 2025-05-08 12:07:40, "Kent Overstreet" <kent.overstreet@linux.dev> wrote:
> >Another thing to note is that memory layout - avoiding pointer chasing -
> >is hugely important, but it'll almost never show up as allocator calls.
> >
> >To give you some examples, mempools and biosets used to be separately
> >allocated. This was mainly to make error paths in outer object
> >constructors/destructors easier and safer: instead of keeping track of
> >what's initialized and what's not, if you've got a pointer to a
> >mempool/bioset you call *_free() on it.
> >
> >(People hadn't yet clued that you can just kzalloc() the entire outer
> >object, and then if the inner object is zeroed it wasn't initialized).
> >
> >But that means you're adding a pointer chase to every mempool_alloc()
> >call, and since bioset itself has mempools allocating bios had _two_
> >unnecessary pointer derefs. That's death for performance when you're
> >running cache cold, but since everyone benchmarks cache-hot...
> >
> >(I was the one who fixed that).
> >
> >Another big one was generic_file_buffered_read(). Main buffered read
> >path, everyone wants it to be as fast as possible.
> >
> >But the core is (was) a loop that walks the pagecache radix tree to get
> >the page, then copies 4k of data out to userspace (there goes l1), then
> >repeats all that pointer chasing for the next 4k. Pre large folios, it
> >was horrific.
> >
> >Solution - vectorize it. Look up all the pages we're copying from all at
> >once, stuff them in a (dynamically allocated! for each read!) vector,
> >and then do the copying out to userspace all at once. Massive
> >performance gain.
> >
> >Of course, to do that I first had to clean up a tangled 250+ line
> >monstrosity of half baked, poorly thought out "optimizations" (the worst
> >spaghetti of gotos you'd ever seen) and turn it into something
> >manageable...
> >
> >So - keep things simple, don't overthink the little stuff, so you can
> >spot and tackle the big algorithmic wins :)
> I will keep this in mind~! :)
>
> And thanks for the enlightening notes~!!
>
> Though I could not quite catch up with the first one, I think I got
> the point: avoid unnecessary pointer chasing and keep the pointer
> chasing as short(balanced) as possible~

To illustrate - DRAM latency is 30-70n.

At 4GHz, that's 120-280 cycles, and a properly fed CPU can do multiple
instructions per clock - so a cache miss all the way to DRAM can cost
you hundreds of instructions.

> The second one, about copy 4k by 4k, seems quite similar to seq_file,
> at least the "4k" part, literally. seq_file read() defaults to alloc
> 4k buffer, and read data until EOF or the 4k buffer is full, and
> start over again for the next read().
>
> One solution could be make changes to seq_file, do not stop until user
> buffer is full for each read. kind of similar to your second note, in
> a sequential style, I think.
>
> If user read with 128K buffer, and seq_file fill the buffer 4k by
> 4k, it would only need ~3 read calls for allocinfo. (I did post a
> patch for seq_file to fill user buffer, but start/stop still happens
> at 4k boundary , so no help for
> the iterator rewinding when read /proc/allocinfo yet.
> https://lore.kernel.org/lkml/20241220140819.9887-1-00107082@163.com/ )
> The solution in this patch is keeping the iterator alive and valid
> cross read boundary, this can also avoid the cost for each start
> over.

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.

The idea of changing seq_file to continue until the user buffer is full
- that'd be a good one, if you're making changes that benefit all
seq_file users.