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Message-ID: <5dc925b6-2faf-4788-9c8c-9387d04417ee@paulmck-laptop>
Date: Thu, 16 Jan 2025 14:54:18 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
rostedt@...dmis.org, Alexei Starovoitov <ast@...nel.org>,
Andrii Nakryiko <andrii@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Kent Overstreet <kent.overstreet@...ux.dev>, bpf@...r.kernel.org
Subject: Re: [PATCH rcu 13/17] srcu: Add SRCU-fast readers
On Thu, Jan 16, 2025 at 01:52:53PM -0800, Andrii Nakryiko wrote:
> On Thu, Jan 16, 2025 at 12:21 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> >
> > This commit adds srcu_read_{,un}lock_fast(), which is similar
> > to srcu_read_{,un}lock_lite(), but avoids the array-indexing and
> > pointer-following overhead. On a microbenchmark featuring tight
> > loops around empty readers, this results in about a 20% speedup
> > compared to RCU Tasks Trace on my x86 laptop.
> >
> > Please note that SRCU-fast has drawbacks compared to RCU Tasks
> > Trace, including:
> >
> > o Lack of CPU stall warnings.
> > o SRCU-fast readers permitted only where rcu_is_watching().
> > o A pointer-sized return value from srcu_read_lock_fast() must
> > be passed to the corresponding srcu_read_unlock_fast().
> > o In the absence of readers, a synchronize_srcu() having _fast()
> > readers will incur the latency of at least two normal RCU grace
> > periods.
> > o RCU Tasks Trace priority boosting could be easily added.
> > Boosting SRCU readers is more difficult.
> >
> > SRCU-fast also has a drawback compared to SRCU-lite, namely that the
> > return value from srcu_read_lock_fast()-fast is a 64-bit pointer and
> > that from srcu_read_lock_lite() is only a 32-bit int.
> >
> > [ paulmck: Apply feedback from Akira Yokosawa. ]
> >
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > Cc: Alexei Starovoitov <ast@...nel.org>
> > Cc: Andrii Nakryiko <andrii@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Kent Overstreet <kent.overstreet@...ux.dev>
> > Cc: <bpf@...r.kernel.org>
> > ---
> > include/linux/srcu.h | 47 ++++++++++++++++++++++++++++++++++++++--
> > include/linux/srcutiny.h | 22 +++++++++++++++++++
> > include/linux/srcutree.h | 38 ++++++++++++++++++++++++++++++++
> > 3 files changed, 105 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index 2bd0e24e9b554..63bddc3014238 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> > @@ -47,9 +47,10 @@ int init_srcu_struct(struct srcu_struct *ssp);
> > #define SRCU_READ_FLAVOR_NORMAL 0x1 // srcu_read_lock().
> > #define SRCU_READ_FLAVOR_NMI 0x2 // srcu_read_lock_nmisafe().
> > #define SRCU_READ_FLAVOR_LITE 0x4 // srcu_read_lock_lite().
> > +#define SRCU_READ_FLAVOR_FAST 0x8 // srcu_read_lock_fast().
> > #define SRCU_READ_FLAVOR_ALL (SRCU_READ_FLAVOR_NORMAL | SRCU_READ_FLAVOR_NMI | \
> > - SRCU_READ_FLAVOR_LITE) // All of the above.
> > -#define SRCU_READ_FLAVOR_SLOWGP SRCU_READ_FLAVOR_LITE
> > + SRCU_READ_FLAVOR_LITE | SRCU_READ_FLAVOR_FAST) // All of the above.
> > +#define SRCU_READ_FLAVOR_SLOWGP (SRCU_READ_FLAVOR_LITE | SRCU_READ_FLAVOR_FAST)
> > // Flavors requiring synchronize_rcu()
> > // instead of smp_mb().
> > void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
> > @@ -253,6 +254,33 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp)
> > return retval;
> > }
> >
> > +/**
> > + * srcu_read_lock_fast - register a new reader for an SRCU-protected structure.
> > + * @ssp: srcu_struct in which to register the new reader.
> > + *
> > + * Enter an SRCU read-side critical section, but for a light-weight
> > + * smp_mb()-free reader. See srcu_read_lock() for more information.
> > + *
> > + * If srcu_read_lock_fast() is ever used on an srcu_struct structure,
> > + * then none of the other flavors may be used, whether before, during,
> > + * or after. Note that grace-period auto-expediting is disabled for _fast
> > + * srcu_struct structures because auto-expedited grace periods invoke
> > + * synchronize_rcu_expedited(), IPIs and all.
> > + *
> > + * Note that srcu_read_lock_fast() can be invoked only from those contexts
> > + * where RCU is watching, that is, from contexts where it would be legal
> > + * to invoke rcu_read_lock(). Otherwise, lockdep will complain.
> > + */
> > +static inline struct srcu_ctr __percpu *srcu_read_lock_fast(struct srcu_struct *ssp) __acquires(ssp)
> > +{
> > + struct srcu_ctr __percpu *retval;
> > +
> > + srcu_check_read_flavor_force(ssp, SRCU_READ_FLAVOR_FAST);
> > + retval = __srcu_read_lock_fast(ssp);
> > + rcu_try_lock_acquire(&ssp->dep_map);
> > + return retval;
> > +}
> > +
> > /**
> > * srcu_read_lock_lite - register a new reader for an SRCU-protected structure.
> > * @ssp: srcu_struct in which to register the new reader.
> > @@ -356,6 +384,21 @@ static inline void srcu_read_unlock(struct srcu_struct *ssp, int idx)
> > __srcu_read_unlock(ssp, idx);
> > }
> >
> > +/**
> > + * srcu_read_unlock_fast - unregister a old reader from an SRCU-protected structure.
> > + * @ssp: srcu_struct in which to unregister the old reader.
> > + * @scp: return value from corresponding srcu_read_lock_fast().
> > + *
> > + * Exit a light-weight SRCU read-side critical section.
> > + */
> > +static inline void srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp)
> > + __releases(ssp)
> > +{
> > + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST);
> > + srcu_lock_release(&ssp->dep_map);
> > + __srcu_read_unlock_fast(ssp, scp);
> > +}
> > +
> > /**
> > * srcu_read_unlock_lite - unregister a old reader from an SRCU-protected structure.
> > * @ssp: srcu_struct in which to unregister the old reader.
> > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > index 07a0c4489ea2f..380260317d98b 100644
> > --- a/include/linux/srcutiny.h
> > +++ b/include/linux/srcutiny.h
> > @@ -71,6 +71,28 @@ static inline int __srcu_read_lock(struct srcu_struct *ssp)
> > return idx;
> > }
> >
> > +struct srcu_ctr;
> > +
> > +static inline bool __srcu_ptr_to_ctr(struct srcu_struct *ssp, struct srcu_ctr __percpu *scpp)
> > +{
> > + return (int)(intptr_t)(struct srcu_ctr __force __kernel *)scpp;
> > +}
> > +
> > +static inline struct srcu_ctr __percpu *__srcu_ctr_to_ptr(struct srcu_struct *ssp, int idx)
> > +{
> > + return (struct srcu_ctr __percpu *)(intptr_t)idx;
> > +}
> > +
> > +static inline struct srcu_ctr __percpu *__srcu_read_lock_fast(struct srcu_struct *ssp)
> > +{
> > + return __srcu_ctr_to_ptr(ssp, __srcu_read_lock(ssp));
> > +}
> > +
> > +static inline void __srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp)
> > +{
> > + __srcu_read_unlock(ssp, __srcu_ptr_to_ctr(ssp, scp));
> > +}
> > +
> > #define __srcu_read_lock_lite __srcu_read_lock
> > #define __srcu_read_unlock_lite __srcu_read_unlock
> >
> > diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> > index ef3065c0cadcd..bdc467efce3a2 100644
> > --- a/include/linux/srcutree.h
> > +++ b/include/linux/srcutree.h
> > @@ -226,6 +226,44 @@ static inline struct srcu_ctr __percpu *__srcu_ctr_to_ptr(struct srcu_struct *ss
> > return &ssp->sda->srcu_ctrs[idx];
> > }
> >
> > +/*
> > + * Counts the new reader in the appropriate per-CPU element of the
> > + * srcu_struct. Returns a pointer that must be passed to the matching
> > + * srcu_read_unlock_fast().
> > + *
> > + * Note that this_cpu_inc() is an RCU read-side critical section either
> > + * because it disables interrupts, because it is a single instruction,
> > + * or because it is a read-modify-write atomic operation, depending on
> > + * the whims of the architecture.
> > + */
> > +static inline struct srcu_ctr __percpu *__srcu_read_lock_fast(struct srcu_struct *ssp)
> > +{
> > + struct srcu_ctr __percpu *scp = READ_ONCE(ssp->srcu_ctrp);
> > +
> > + RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_fast().");
> > + this_cpu_inc(scp->srcu_locks.counter); /* Y */
> > + barrier(); /* Avoid leaking the critical section. */
> > + return scp;
> > +}
> > +
> > +/*
> > + * Removes the count for the old reader from the appropriate
> > + * per-CPU element of the srcu_struct. Note that this may well be a
> > + * different CPU than that which was incremented by the corresponding
> > + * srcu_read_lock_fast(), but it must be within the same task.
>
> hm... why the "same task" restriction? With uretprobes we take
> srcu_read_lock under a traced task, but we can "release" this lock
> from timer interrupt, which could be in the context of any task.
A kneejerk reaction on my part? ;-)
But the good news is that this restriction is easy for me to relax.
I adjust the comments and remove the rcu_try_lock_acquire()
from srcu_read_lock_fast() and the srcu_lock_release() from
srcu_read_unlock_fast().
But in that case, I should rename this to srcu_down_read_fast() and
srcu_up_read_fast() or similar, as these names would clearly indicate
that they can be used cross-task. (And from interrupt handlers, but
not from NMI handlers.)
Also, srcu_read_lock() and srcu_read_unlock() have srcu_lock_acquire() and
srcu_lock_release() calls, so I am surprised that lockdep didn't complain
when you invoked srcu_read_lock() from task context and srcu_read_unlock()
from a timer interrupt.
Thanx, Paul
> > + *
> > + * Note that this_cpu_inc() is an RCU read-side critical section either
> > + * because it disables interrupts, because it is a single instruction,
> > + * or because it is a read-modify-write atomic operation, depending on
> > + * the whims of the architecture.
> > + */
> > +static inline void __srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp)
> > +{
> > + barrier(); /* Avoid leaking the critical section. */
> > + this_cpu_inc(scp->srcu_unlocks.counter); /* Z */
> > + RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_fast().");
> > +}
> > +
> > /*
> > * Counts the new reader in the appropriate per-CPU element of the
> > * srcu_struct. Returns an index that must be passed to the matching
> > --
> > 2.40.1
> >
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