Re: [RFC PATCH 00/16] mm/madvise: batch tlb flushes for MADV_DONTNEED and MADV_FREE

[SeongJae Park <sj@kernel.org>]

On Wed, 5 Mar 2025 12:22:25 -0800 Shakeel Butt <shakeel.butt@linux.dev> wrote:

> On Wed, Mar 05, 2025 at 10:15:55AM -0800, SeongJae Park wrote:
> > For MADV_DONTNEED[_LOCKED] or MADV_FREE madvise requests, tlb flushes
> > can happen for each vma of the given address ranges.  Because such tlb
> > flushes are for address ranges of same process, doing those in a batch
> > is more efficient while still being safe.  Modify madvise() and
> > process_madvise() entry level code path to do such batched tlb flushes,
> > while the internal unmap logics do only gathering of the tlb entries to
> > flush.
> > 
> > In more detail, modify the entry functions to initialize an mmu_gather
> > ojbect and pass it to the internal logics.  Also modify the internal
> > logics to do only gathering of the tlb entries to flush into the
> > received mmu_gather object.  After all internal function calls are done,
> > the entry functions finish the mmu_gather object to flush the gathered
> > tlb entries in the one batch.
> > 
> > Patches Seuquence
> > =================
> > 
> > First four patches are minor cleanups of madvise.c for readability.
> > 
> > Following four patches (patches 5-8) define new data structure for
> > managing information that required for batched tlb flushing (mmu_gather
> > and behavior), and update code paths for MADV_DONTNEED[_LOCKED] and
> > MADV_FREE handling internal logics to receive it.
> > 
> > Three patches (patches 9-11) for making internal MADV_DONTNEED[_LOCKED]
> > and MADV_FREE handling logic ready for batched tlb flushing follow. 
> 
> I think you forgot to complete the above sentence or the 'follow' at the
> end seems weird.

Thank you for catching this.  I just wanted to say these three patches come
after the previous ones.  I will wordsmith this part in the next version.

> 
> > The
> > patches keep the support of unbatched tlb flushes use case, for
> > fine-grained and safe transitions.
> > 
> > Next three patches (patches 12-14) update madvise() and
> > process_madvise() code to do the batched tlb flushes utilizing the
> > previous patches introduced changes.
> > 
> > Final two patches (patches 15-16) clean up the internal logics'
> > unbatched tlb flushes use case support code, which is no more be used.
> > 
> > Test Results
> > ============
> > 
> > I measured the time to apply MADV_DONTNEED advice to 256 MiB memory
> > using multiple process_madvise() calls.  I apply the advice in 4 KiB
> > sized regions granularity, but with varying batch size (vlen) from 1 to
> > 1024.  The source code for the measurement is available at GitHub[1].
> > 
> > The measurement results are as below.  'sz_batches' column shows the
> > batch size of process_madvise() calls.  'before' and 'after' columns are
> > the measured time to apply MADV_DONTNEED to the 256 MiB memory buffer in
> > nanoseconds, on kernels that built without and with the MADV_DONTNEED
> > tlb flushes batching patch of this series, respectively.  For the
> > baseline, mm-unstable tree of 2025-03-04[2] has been used.
> > 'after/before' column is the ratio of 'after' to 'before'.  So
> > 'afetr/before' value lower than 1.0 means this patch increased
> > efficiency over the baseline.  And lower value means better efficiency.
> 
> I would recommend to replace the after/end column with percentage i.e.
> percentage improvement or degradation.

Thank you for the nice suggestion.  I will do so in the next version.

> 
> > 
> >     sz_batches    before       after        after/before
> >     1             102842895    106507398    1.03563204828102
> >     2             73364942     74529223     1.01586971880929
> >     4             58823633     51608504     0.877343022998937
> >     8             47532390     44820223     0.942940655834895
> >     16            43591587     36727177     0.842529018271347
> >     32            44207282     33946975     0.767904595446515
> >     64            41832437     26738286     0.639175910310939
> >     128           40278193     23262940     0.577556694263817
> >     256           41568533     22355103     0.537789077136785
> >     512           41626638     22822516     0.54826709762148
> >     1024          44440870     22676017     0.510251419470411
> > 
> > For <=2 batch size, tlb flushes batching shows no big difference but
> > slight overhead.  I think that's in an error range of this simple
> > micro-benchmark, and therefore can be ignored.  
> 
> I would recommend to run the experiment multiple times and report
> averages and standard deviation which will support your error range
> claim.

Again, good suggestion.  I will do so.

> 
> > Starting from batch size
> > 4, however, tlb flushes batching shows clear efficiency gain.  The
> > efficiency gain tends to be proportional to the batch size, as expected.
> > The efficiency gain ranges from about 13 percent with batch size 4, and
> > up to 49 percent with batch size 1,024.
> > 
> > Please note that this is a very simple microbenchmark, so real
> > efficiency gain on real workload could be very different.
> > 
> 
> I think you are running a single thread benchmark on a free machine. I
> expect this series to be much more beneficial on loaded machine and for
> multi-threaded applications.

Your understanding of my test setup is correct and I agree to your expectation.

> No need to test that scenario but if you
> have already done that then it would be good to report.

I don't have such test results or plans for those with specific timeline for
now.  I will share those if I get a chance, of course.


Thanks,
SJ