Re: [PATCH] rcu: Merge rcu_seq_done_exact() logic into rcu_seq_done()

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, Jan 27, 2025 at 07:09:34PM -0500, Joel Fernandes wrote:
> On Mon, Jan 27, 2025 at 7:07 PM Joel Fernandes (Google)
> <joel@joelfernandes.org> wrote:
> >
> > The rcu_seq_done() API has a large "false-negative" windows of size
> > ULONG_MAX/2, where after wrap around, it is possible that it will think
> > that a GP has not completed if a wrap around happens and the delta is
> > large.
> >
> > rcu_seq_done_exact() is more accurate avoiding this wrap around issue,
> > by making the window of false-negativity by only 3 GPs. Use this logic
> > for rcu_seq_done() which is a nice negative code delta and could
> > potentially avoid issues in the future where rcu_seq_done() was
> > reporting false-negatives for too long.
> >
> > rcutorture runs of all scenarios for 15 minutes passed. Code inspection
> > was done of all users to convince the change would work.
> >
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> 
> I am leaving a 60 minute overnight run of all scenarios on my personal
> server for further testing.

The run passed, details below:

 --- Mon Jan 27 11:49:49 PM EST 2025 Test summary:
Results directory:
tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 60
RUDE01 ------- 14309 GPs (3.97472/s) [tasks-rude: g57884 f0x0 total-gps=57880] n_max_cbs: 0
SRCU-L ------- 34121 GPs (9.47806/s) [srcu: g316564 f0x0 total-gps=79242] n_max_cbs: 150000
SRCU-N ------- 151316 GPs (42.0322/s) [srcu: g1840064 f0x0 total-gps=460117] n_max_cbs: 150000
SRCU-P ------- 35189 GPs (9.77472/s) [srcud: g320792 f0x0 total-gps=80299] n_max_cbs: 150000
SRCU-T ------- 389034 GPs (108.065/s) [srcu: g4142406 f0x0 total-gps=1035602] n_max_cbs: 50000
SRCU-U ------- 376267 GPs (104.519/s) [srcud: g3953834 f0x0 total-gps=988459] n_max_cbs: 50000
SRCU-V ------- 407633 GPs (113.231/s) [srcud: g4371704 f0x0 total-gps=1092927] n_max_cbs: 1000
TASKS01 ------- 11007 GPs (3.0575/s) [tasks: g57816 f0x0 total-gps=57808]
TASKS02 ------- 10539 GPs (2.9275/s) [tasks: g57936 f0x0 total-gps=57936]
TASKS03 ------- 10453 GPs (2.90361/s) [tasks: g57508 f0x0 total-gps=57508]
TINY01 ------- 511634 GPs (142.121/s) [rcu: g0 f0x0 total-gps=0] n_max_cbs: 57078
TINY02 ------- 541799 GPs (150.5/s) [rcu: g0 f0x0 total-gps=0] n_max_cbs: 2619
TRACE01 ------- 7299 GPs (2.0275/s) [tasks-tracing: g45844 f0x0 total-gps=45844] n_max_cbs: 50000
TRACE02 ------- 101265 GPs (28.1292/s) [tasks-tracing: g305464 f0x0 total-gps=305456] n_max_cbs: 100000
TREE01 ------- 97989 GPs (27.2192/s) [rcu: g479473 f0x0 total-gps=120151]
TREE02 ------- 202908 GPs (56.3633/s) [rcu: g1459509 f0x0 total-gps=365162] n_max_cbs: 1139244
TREE03 ------- 168901 GPs (46.9169/s) [rcu: g1764445 f0x0 total-gps=441393] n_max_cbs: 1341765
TREE04 ------- 148876 GPs (41.3544/s) [rcu: g951744 f0x0 total-gps=238225] n_max_cbs: 236765
TREE05 ------- 220092 GPs (61.1367/s) [rcu: g1234385 f0x0 total-gps=308880] n_max_cbs: 82801
TREE07 ------- 34678 GPs (9.63278/s) [rcu: g207257 f0x0 total-gps=52094]
TREE09 ------- 341706 GPs (94.9183/s) [rcu: g7693569 f0x0 total-gps=1923688] n_max_cbs: 1845334
 --- Done at Mon Jan 27 11:49:55 PM EST 2025 (4:41:24) exitcode 0

thanks,

 - Joel


> 
> thanks,
> 
>  - Joel
> 
> 
> 
> 
> > ---
> >  kernel/rcu/rcu.h  | 13 ++-----------
> >  kernel/rcu/tree.c |  6 +++---
> >  2 files changed, 5 insertions(+), 14 deletions(-)
> >
> > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > index eed2951a4962..c2ca196907cb 100644
> > --- a/kernel/rcu/rcu.h
> > +++ b/kernel/rcu/rcu.h
> > @@ -146,19 +146,10 @@ static inline bool rcu_seq_started(unsigned long *sp, unsigned long s)
> >
> >  /*
> >   * Given a snapshot from rcu_seq_snap(), determine whether or not a
> > - * full update-side operation has occurred.
> > + * full update-side operation has occurred while also handling
> > + * wraparounds that exceed the (ULONG_MAX / 2) safety-factor/guard-band.
> >   */
> >  static inline bool rcu_seq_done(unsigned long *sp, unsigned long s)
> > -{
> > -       return ULONG_CMP_GE(READ_ONCE(*sp), s);
> > -}
> > -
> > -/*
> > - * Given a snapshot from rcu_seq_snap(), determine whether or not a
> > - * full update-side operation has occurred, but do not allow the
> > - * (ULONG_MAX / 2) safety-factor/guard-band.
> > - */
> > -static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s)
> >  {
> >         unsigned long cur_s = READ_ONCE(*sp);
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index b77ccc55557b..835600cec9ba 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -4300,7 +4300,7 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_full);
> >  bool poll_state_synchronize_rcu(unsigned long oldstate)
> >  {
> >         if (oldstate == RCU_GET_STATE_COMPLETED ||
> > -           rcu_seq_done_exact(&rcu_state.gp_seq_polled, oldstate)) {
> > +           rcu_seq_done(&rcu_state.gp_seq_polled, oldstate)) {
> >                 smp_mb(); /* Ensure GP ends before subsequent accesses. */
> >                 return true;
> >         }
> > @@ -4347,9 +4347,9 @@ bool poll_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
> >
> >         smp_mb(); // Order against root rcu_node structure grace-period cleanup.
> >         if (rgosp->rgos_norm == RCU_GET_STATE_COMPLETED ||
> > -           rcu_seq_done_exact(&rnp->gp_seq, rgosp->rgos_norm) ||
> > +           rcu_seq_done(&rnp->gp_seq, rgosp->rgos_norm) ||
> >             rgosp->rgos_exp == RCU_GET_STATE_COMPLETED ||
> > -           rcu_seq_done_exact(&rcu_state.expedited_sequence, rgosp->rgos_exp)) {
> > +           rcu_seq_done(&rcu_state.expedited_sequence, rgosp->rgos_exp)) {
> >                 smp_mb(); /* Ensure GP ends before subsequent accesses. */
> >                 return true;
> >         }
> > --
> > 2.34.1
> >