This commit documents the implicit RCU readers that are implied by the
this_cpu_inc() and atomic_long_inc() operations in __srcu_read_lock_fast()
and __srcu_read_unlock_fast(). While in the area, fix the documentation
of the memory pairing of atomic_long_inc() in __srcu_read_lock_fast().
[ paulmck: Apply Joel Fernandes feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: <bpf@vger.kernel.org>
The rcu_is_watching() warnings are currently in the SRCU-tree
implementations of __srcu_read_lock_fast() and __srcu_read_unlock_fast().
However, this makes it difficult to create _notrace variants of
srcu_read_lock_fast() and srcu_read_unlock_fast(). This commit therefore
moves these checks to srcu_read_lock_fast(), srcu_read_unlock_fast(),
srcu_down_read_fast(), and srcu_up_read_fast().
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: <bpf@vger.kernel.org>
This commit removes the SRCU-lite implementation, which has been replaced
by SRCU-fast.
Both SRCU-lite and SRCU-fast provide faster readers by dropping the
smp_mb() call from their lock and unlock primitives, but incur a pair
of added RCU grace periods during the SRCU grace period. There is a
trivial mapping from the SRCU-lite API to that of SRCU-fast, so there
should be no transition issues.
[ paulmck: Apply Christoph Hellwig feedback. ]
Signed-off-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>
BPF uses rcu_read_lock_trace() in NMI context, so srcu_read_lock_fast()
must be NMI-safe if it is to have any chance of addressing RCU Tasks
Trace use cases. This commit therefore causes srcu_read_lock_fast()
and srcu_read_unlock_fast() to use atomic_long_inc() instead of
this_cpu_inc() on architectures that support NMIs but do not have
NMI-safe implementations of this_cpu_inc(). Note that both x86 and
arm64 have NMI-safe implementations of this_cpu_inc(), and thus do not
pay the performance penalty inherent in atomic_inc_long().
It is tempting to use this trick to fold srcu_read_lock_nmisafe()
into srcu_read_lock(), but this would need careful thought, review,
and performance analysis. Though those smp_mb() calls might well make
performance a non-issue.
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
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@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
There are a couple of definitions under "#ifdef CONFIG_TINY_SRCU"
in include/linux/srcu.h. There is no point in them being there,
so this commit moves them to include/linux/srcutiny.h and
include/linux/srcutree.c, thus eliminating that #ifdef.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
This commit abstracts the srcu_read_unlock*() integer-to-pointer
conversion into a new __srcu_ctr_to_ptr(). This will be used
in rcutorture for testing an srcu_read_unlock_fast() that avoids
array-indexing overhead by taking a pointer rather than an integer.
[ paulmck: Apply kernel test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
This commit renames the srcu_check_read_flavor_lite() function to
srcu_check_read_flavor_force() and adds a read_flavor argument in order to
support an srcu_read_lock_fast() variant that is to avoid array indexing
in both the lock and unlock primitives.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Currently, srcu_read_lock_lite() uses the SRCU_READ_FLAVOR_LITE bit in
->srcu_reader_flavor to communicate to the grace-period processing in
srcu_readers_active_idx_check() that the smp_mb() must be replaced by a
synchronize_rcu(). Unfortunately, ->srcu_reader_flavor is not updated
unless the kernel is built with CONFIG_PROVE_RCU=y. Therefore in all
kernels built with CONFIG_PROVE_RCU=n, srcu_readers_active_idx_check()
incorrectly uses smp_mb() instead of synchronize_rcu() for srcu_struct
structures whose readers use srcu_read_lock_lite().
This commit therefore causes Tree SRCU srcu_read_lock_lite()
to unconditionally update ->srcu_reader_flavor so that
srcu_readers_active_idx_check() can make the correct choice.
Reported-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Closes: https://lore.kernel.org/all/d07e8f4a-d5ff-4c8e-8e61-50db285c57e9@amd.com/
Fixes: c0f08d6b5a61 ("srcu: Add srcu_read_lock_lite() and srcu_read_unlock_lite()")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
This patch adds srcu_read_lock_lite() and srcu_read_unlock_lite(), which
dispense with the read-side smp_mb() but also are restricted to code
regions that RCU is watching. If a given srcu_struct structure uses
srcu_read_lock_lite() and srcu_read_unlock_lite(), it is not permitted
to use any other SRCU read-side marker, before, during, or after.
Another price of light-weight readers is heavier weight grace periods.
Such readers mean that SRCU grace periods on srcu_struct structures
used by light-weight readers will incur at least two calls to
synchronize_rcu(). In addition, normal SRCU grace periods for
light-weight-reader srcu_struct structures never auto-expedite.
Note that expedited SRCU grace periods for light-weight-reader
srcu_struct structures still invoke synchronize_rcu(), not
synchronize_srcu_expedited(). Something about wishing to keep
the IPIs down to a dull roar.
The srcu_read_lock_lite() and srcu_read_unlock_lite() functions may not
(repeat, *not*) be used from NMI handlers, but if this is needed, an
additional flavor of SRCU reader can be added by some future commit.
[ paulmck: Apply Alexei Starovoitov expediting feedback. ]
[ paulmck: Apply kernel test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: kernel test robot <oliver.sang@intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
This commit creates SRCU_READ_FLAVOR_NORMAL and SRCU_READ_FLAVOR_NMI
C-preprocessor macros for srcu_read_lock() and srcu_read_lock_nmisafe(),
respectively. These replace the old true/false values that were
previously passed to srcu_check_read_flavor(). In addition, the
srcu_check_read_flavor() function itself requires a bit of rework to
handle bitmasks instead of true/false values.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Currently, there are only two flavors of readers, normal and NMI-safe.
A number of fields, functions, and types reflect this restriction.
This renaming-only commit prepares for the addition of light-weight
(as in memory-barrier-free) readers. OK, OK, there is also a drive-by
white-space fixeup!
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Using a higher value for the initial gp sequence counters allows for
wrapping to occur faster. It can help with surfacing any issues that may
be happening as a result of the wrap around.
Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
This commit moves the ->reschedule_jiffies, ->reschedule_count, and
->work fields from the srcu_struct structure to the srcu_usage structure
to reduce the size of the former in order to improve cache locality.
However, this means that the container_of() calls cannot get a pointer
to the srcu_struct because they are no longer in the srcu_struct.
This issue is addressed by adding a ->srcu_ssp field in the srcu_usage
structure that references the corresponding srcu_struct structure.
And given the presence of the sup pointer to the srcu_usage structure,
replace some ssp->srcu_usage-> instances with sup->.
[ paulmck Apply feedback from kernel test robot. ]
Link: https://lore.kernel.org/oe-kbuild-all/202303191400.iO5BOqka-lkp@intel.com/
Suggested-by: Christoph Hellwig <hch@lst.de>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Tested-by: "Zhang, Qiang1" <qiang1.zhang@intel.com>
Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Paul E. McKenney
Uladzislau Rezki (Sony)
JP Kobryn
Joel Fernandes (Google)
Pingfan Liu