Re: [RFC PATCH v2 3/4] hp: Implement Hazard Pointers

[Mathieu Desnoyers]

On 2024-10-04 23:25, Joel Fernandes wrote:
> On Fri, Oct 4, 2024 at 2:29 PM Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> > This API provides existence guarantees of objects through Hazard
> > Pointers (HP). This minimalist implementation is specific to use
> > with preemption disabled, but can be extended further as needed.
> >
> > Each HP domain defines a fixed number of hazard pointer slots (nr_cpus)
> > across the entire system.
> >
> > Its main benefit over RCU is that it allows fast reclaim of
> > HP-protected pointers without needing to wait for a grace period.
> >
> > It also allows the hazard pointer scan to call a user-defined callback
> > to retire a hazard pointer slot immediately if needed. This callback
> > may, for instance, issue an IPI to the relevant CPU.
> >
> > There are a few possible use-cases for this in the Linux kernel:
> >
> >    - Improve performance of mm_count by replacing lazy active mm by HP.
> >    - Guarantee object existence on pointer dereference to use refcount:
> >      - replace locking used for that purpose in some drivers,
> >      - replace RCU + inc_not_zero pattern,
> >    - rtmutex: Improve situations where locks need to be taken in
> >      reverse dependency chain order by guaranteeing existence of
> >      first and second locks in traversal order, allowing them to be
> >      locked in the correct order (which is reverse from traversal
> >      order) rather than try-lock+retry on nested lock.
> >
> > References:
> >
> > [1]: M. M. Michael, "Hazard pointers: safe memory reclamation for
> >       lock-free objects," in IEEE Transactions on Parallel and
> >       Distributed Systems, vol. 15, no. 6, pp. 491-504, June 2004
> [ ... ]
> > ---
> > Changes since v0:
> > - Remove slot variable from hp_dereference_allocate().
> > ---
> >   include/linux/hp.h | 158 +++++++++++++++++++++++++++++++++++++++++++++
> >   kernel/Makefile    |   2 +-
> >   kernel/hp.c        |  46 +++++++++++++
>
> Just a housekeeping comment, ISTR Linus looking down on adding bodies
> of C code to header files (like hp_dereference_allocate). I understand
> maybe the rationale is that the functions included are inlined. But do
> all of them have to be inlined? Such headers also hurt code browsing
> capabilities in code browsers like clangd. clangd doesn't understand
> header files because it can't independently compile them -- it uses
> the compiler to generate and extract the AST for superior code
> browsing/completion.
>
> Also have you looked at the benefits of inlining for hp.h?
> hp_dereference_allocate() seems large enough that inlining may not
> matter much, but I haven't compiled it and looked at the asm myself.

Here is a comparison in userspace:

* With "hp dereference allocate" inlined:

    test_hpref_benchmark (smp_mb)             nr_reads   1994298193 nr_writes     22293162 nr_ops   2016591355
    test_hpref_benchmark (barrier/membarrier) nr_reads  15208690879 nr_writes      1893785 nr_ops  15210584664

* With "hp dereference allocate" implemented as a function call:

    test_hpref_benchmark (smp_mb)             nr_reads   1558924716 nr_writes     14261028 nr_ops   1573185744
    test_hpref_benchmark (barrier/membarrier) nr_reads   5881131707 nr_writes      2005140 nr_ops   5883136847

So the overhead of the function call when using symmetric memory barriers
between hp allocate/hp scan is a 20% slowdown.

It's worse in the asymmetric barrier/membarrier case, introducing a 61%
slowdown.

Given that the overhead is noticeable, I am tempted to leave the hazard
pointer allocate/retire as inline functions.

About code browsers like clangd, I would recommend improving the tooling
rather than alter the design of the code based on current tooling
limitations.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com