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atomic_t based reference counting, including refcount_t, uses
atomic_inc_not_zero() for acquiring a reference. atomic_inc_not_zero() is
implemented with a atomic_try_cmpxchg() loop. High contention of the
reference count leads to retry loops and scales badly. There is nothing to
improve on this implementation as the semantics have to be preserved.
Provide rcuref as a scalable alternative solution which is suitable for RCU
managed objects. Similar to refcount_t it comes with overflow and underflow
detection and mitigation.
rcuref treats the underlying atomic_t as an unsigned integer and partitions
this space into zones:
0x00000000 - 0x7FFFFFFF valid zone (1 .. (INT_MAX + 1) references)
0x80000000 - 0xBFFFFFFF saturation zone
0xC0000000 - 0xFFFFFFFE dead zone
0xFFFFFFFF no reference
rcuref_get() unconditionally increments the reference count with
atomic_add_negative_relaxed(). rcuref_put() unconditionally decrements the
reference count with atomic_add_negative_release().
This unconditional increment avoids the inc_not_zero() problem, but
requires a more complex implementation on the put() side when the count
drops from 0 to -1.
When this transition is detected then it is attempted to mark the reference
count dead, by setting it to the midpoint of the dead zone with a single
atomic_cmpxchg_release() operation. This operation can fail due to a
concurrent rcuref_get() elevating the reference count from -1 to 0 again.
If the unconditional increment in rcuref_get() hits a reference count which
is marked dead (or saturated) it will detect it after the fact and bring
back the reference count to the midpoint of the respective zone. The zones
provide enough tolerance which makes it practically impossible to escape
from a zone.
The racy implementation of rcuref_put() requires to protect rcuref_put()
against a grace period ending in order to prevent a subtle use after
free. As RCU is the only mechanism which allows to protect against that, it
is not possible to fully replace the atomic_inc_not_zero() based
implementation of refcount_t with this scheme.
The final drop is slightly more expensive than the atomic_dec_return()
counterpart, but that's not the case which this is optimized for. The
optimization is on the high frequeunt get()/put() pairs and their
scalability.
The performance of an uncontended rcuref_get()/put() pair where the put()
is not dropping the last reference is still on par with the plain atomic
operations, while at the same time providing overflow and underflow
detection and mitigation.
The performance of rcuref compared to plain atomic_inc_not_zero() and
atomic_dec_return() based reference counting under contention:
- Micro benchmark: All CPUs running a increment/decrement loop on an
elevated reference count, which means the 0 to -1 transition never
happens.
The performance gain depends on microarchitecture and the number of
CPUs and has been observed in the range of 1.3X to 4.7X
- Conversion of dst_entry::__refcnt to rcuref and testing with the
localhost memtier/memcached benchmark. That benchmark shows the
reference count contention prominently.
The performance gain depends on microarchitecture and the number of
CPUs and has been observed in the range of 1.1X to 2.6X over the
previous fix for the false sharing issue vs. struct
dst_entry::__refcnt.
When memtier is run over a real 1Gb network connection, there is a
small gain on top of the false sharing fix. The two changes combined
result in a 2%-5% total gain for that networked test.
Reported-by: Wangyang Guo <wangyang.guo@intel.com>
Reported-by: Arjan Van De Ven <arjan.van.de.ven@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230323102800.158429195@linutronix.de
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