Re: [PATCH v2 0/4] mm: introduce mthp_ext via cgroup-bpf to make mTHP more transparent

[Lorenzo Stoakes <ljs@kernel.org>]

On Fri, May 08, 2026 at 05:00:04PM +0100, Pedro Falcato wrote:
> On Fri, May 08, 2026 at 11:00:51PM +0800, Vernon Yang wrote:
> > From: Vernon Yang <yanglincheng@kylinos.cn>
> >
> > Hi all,
> >
> > Background
> > ==========
> >
> > As is well known, a system can simultaneously run multiple different
> > scenarios. However, THP is not beneficial in every scenario — it is only
> > most suitable for memory-intensive applications that are not sensitive
> > to tail latency. For example, Redis, which is sensitive to tail latency,
> > is not suitable for THP. But in practice, due to Redis issues, the
> > entire THP functionality is often turned off, preventing other scenarios
> > from benefiting from it.
> >
> > There are also some embedded scenarios (e.g. Android) that directly use
> > 2MB THP, where the granularity is too large. Therefore, we introduced
> > mTHP in v6.8, which supports multiple-size THP. In practice, however, we
> > still globally fix a single mTHP size and are unable to automatically
> > select different mTHP sizes based on different scenarios.
> >
> > After testing, it was found that
> >
> > - When the system has a lot of free memory, it is normal for Redis to
> >   use mTHP. performance degradation in Redis only occurs when the system
> >   is under high memory pressure.
> > - Additionally, when a large number of small-memory processes use mTHP,
> >   memory waste is prone to occur, and performance degradation may also
> >   happen during fast memory allocation/release.
> >
> > Previously, "Cgroup-based THP control"[1] was proposed, but it had the
> > following issues.
> >
> > - It breaks the cgroup hierarchy property.
> > - Add new THP knobs, making sysadmin's job more complex
> >
> > Previously, "mm, bpf: BPF-MM, BPF-THP"[2] was proposed, but it had the
> > following issues.
> >
> > - It didn't address the issue on the per-process mode.
> > - For global mode, the prctl(PR_SET_THP_DISABLE) has already achieved
> >   the same objective, there is no need to add two mechanisms for the
> >   same purpose.
> > - Attaching st_ops to mm_struct, the same issues that cgroup-bpf once
> >   faced are likely to arise again, e.g. lifetime of cgroup vs bpf, dying
> >   cgroups, wq deadlock, etc. It is recommended to use cgroup-bpf for
> >   implementation.
> > - Unclear ABI stability guarantees.
> > - The test cases are too simplistic, lacking eBPF cases similar to real
> >   workloads such as sched_ext.
> >
> > If I miss some thing, please let me know. Thanks!
> >
> <snip>
> > kernbench results
> > ~~~~~~~~~~~~~~~~~
> >
> > When cgroup memory.high=max, no memory pressure, seems only noise level
> > changes, mthp_ext no regression.
> >
> >                             always                 never               always+mthp_ext
> > Amean     user-32    19702.39 (   0.00%)    18428.90 *   6.46%*    19706.73 (  -0.02%)
> > Amean     syst-32     1159.55 (   0.00%)     2252.43 * -94.25%*     1177.48 *  -1.55%*
> > Amean     elsp-32      703.28 (   0.00%)      699.10 *   0.59%*      703.99 *  -0.10%*
> > BAmean-95 user-32    19701.79 (   0.00%)    18425.01 (   6.48%)    19704.78 (  -0.02%)
> > BAmean-95 syst-32     1159.43 (   0.00%)     2251.86 ( -94.22%)     1177.03 (  -1.52%)
> > BAmean-95 elsp-32      703.24 (   0.00%)      698.99 (   0.61%)      703.88 (  -0.09%)
> > BAmean-99 user-32    19701.79 (   0.00%)    18425.01 (   6.48%)    19704.78 (  -0.02%)
> > BAmean-99 syst-32     1159.43 (   0.00%)     2251.86 ( -94.22%)     1177.03 (  -1.52%)
> > BAmean-99 elsp-32      703.24 (   0.00%)      698.99 (   0.61%)      703.88 (  -0.09%)
> >
> > When cgroup memory.high=2G, high memory pressure, mthp_ext improved by 26%.
> >
> >                             always                 never               always+mthp_ext
> > Amean     user-32    20250.65 (   0.00%)    18368.91 *   9.29%*    18681.27 *   7.75%*
> > Amean     syst-32    12778.56 (   0.00%)     9636.99 *  24.58%*     9392.65 *  26.50%*
> > Amean     elsp-32     1377.55 (   0.00%)     1026.10 *  25.51%*     1019.40 *  26.00%*
> > BAmean-95 user-32    20233.75 (   0.00%)    18353.57 (   9.29%)    18678.01 (   7.69%)
> > BAmean-95 syst-32    12543.21 (   0.00%)     9612.28 (  23.37%)     9386.83 (  25.16%)
> > BAmean-95 elsp-32     1367.82 (   0.00%)     1023.75 (  25.15%)     1018.17 (  25.56%)
> > BAmean-99 user-32    20233.75 (   0.00%)    18353.57 (   9.29%)    18678.01 (   7.69%)
> > BAmean-99 syst-32    12543.21 (   0.00%)     9612.28 (  23.37%)     9386.83 (  25.16%)
> > BAmean-99 elsp-32     1367.82 (   0.00%)     1023.75 (  25.15%)     1018.17 (  25.56%)
> >
> > TODO
> > ====
> >
> > - mthp_ext handles different "enum tva_type" values. For example, for
> >   small-memory processes, only 4KB is used in TVA_PAGEFAULT, while
> >   TVA_KHUGEPAGED/TVA_FORCED_COLLAPSE continues to collapse all mthp
> >   size. Under high memory pressure, only 4KB is used for
> >   TVA_PAGEFAULT/TVA_KHUGEPAGED, while TVA_FORCED_COLLAPSE continues to
> >   collapse all mthp size.
> > - selftest
> >
> > If there are additional scenarios, please let me know as well, so I can
> > conduct further prototype verification tests to make mTHP more
> > transparent and further clear/stabilize the BPF-THP ABI.
>
> How is it more transparent if you're essentially adding mTHP
> micro-programmability from the user's side? This series makes it
> _less_ transparent.
>
> If you actually want to make it more transparent, then I would suggest
> improving the heuristics such that (m)THP doesn't churn through memory
> on high memory pressure. Or such that it doesn't feel extremely compelled
> to place the largest THP it can based on vibes.

I agree but I also don't really want to see anything like that until mTHP is
actually stabilised and the code base is less appalling :)

We've deferred paying down technical debt far too long.

>
> --
> Pedro

Thanks, Lorenzo