Re: [PATCH] mm/readahead: Skip fully overlapped range

[Aubrey Li <aubrey.li@linux.intel.com>]

On 9/23/25 17:57, Jan Kara wrote:
> On Tue 23-09-25 13:11:37, Aubrey Li wrote:
>> On 9/23/25 11:49, Andrew Morton wrote:
>>> On Tue, 23 Sep 2025 11:59:46 +0800 Aubrey Li <aubrey.li@linux.intel.com> wrote:
>>>
>>>> RocksDB sequential read benchmark under high concurrency shows severe
>>>> lock contention. Multiple threads may issue readahead on the same file
>>>> simultaneously, which leads to heavy contention on the xas spinlock in
>>>> filemap_add_folio(). Perf profiling indicates 30%~60% of CPU time spent
>>>> there.
>>>>
>>>> To mitigate this issue, a readahead request will be skipped if its
>>>> range is fully covered by an ongoing readahead. This avoids redundant
>>>> work and significantly reduces lock contention. In one-second sampling,
>>>> contention on xas spinlock dropped from 138,314 times to 2,144 times,
>>>> resulting in a large performance improvement in the benchmark.
>>>>
>>>> 				w/o patch       w/ patch
>>>> RocksDB-readseq (ops/sec)
>>>> (32-threads)			1.2M		2.4M
>>>
>>> On which kernel version?  In recent times we've made a few readahead
>>> changes to address issues with high concurrency and a quick retest on
>>> mm.git's current mm-stable branch would be interesting please.
>>
>> I'm on v6.16.7. Thanks Andrew for the information, let me check with mm.git.
> 
> I don't expect much of a change for this load but getting test result with
> mm.git as a confirmation would be nice. 

Yes, the hotspot remains on mm.git:mm-stable branch.

   - 88.68% clone3
      - 88.68% start_thread
         - 88.68% reader_thread
            - 88.27% syscall
                 entry_SYSCALL_64_after_hwframe
                 do_syscall_64
                 ksys_readahead
                 generic_fadvise
                 force_page_cache_ra
                 page_cache_ra_unbounded
                 filemap_add_folio
                 __filemap_add_folio
                 _raw_spin_lock_irq
               - do_raw_spin_lock
                    native_queued_spin_lock_slowpath

> Also, based on the fact that the
> patch you propose helps, this looks like there are many threads sharing one
> struct file which race to read the same content. That is actually rather
> problematic for current readahead code because there's *no synchronization*
> on updating file's readhead state. So threads can race and corrupt the
> state in interesting ways under one another's hands. On rare occasions I've
> observed this with heavy NFS workload where the NFS server is
> multithreaded. Since the practical outcome is "just" reduced read
> throughput / reading too much, it was never high enough on my priority list
> to fix properly (I do have some preliminary patch for that laying around
> but there are some open questions that require deeper thinking - like how
> to handle a situation where one threads does readahead, filesystem requests
> some alignment of the request size after the fact, so we'd like to update
> readahead state but another thread has modified the shared readahead state
> in the mean time).  But if we're going to work on improving behavior of
> readahead for multiple threads sharing readahead state, fixing the code so
> that readahead state is at least consistent is IMO the first necessary
> step. And then we can pile more complex logic on top of that.

This makes sense. I actually had a version using atomic operations to update
ra in my patch, but I found that ra is also updated in other paths without
synchronization, so I dropped the atomic operations before sending the patch.
Let me check what I can do for this.

Have you put your preliminary patch somewhere?

Thanks,
-Aubrey
> 
> 								Honza