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Message-ID: <26cddadd-a79b-47b1-923e-9684cd8a7ef4@paulmck-laptop> Date: Tue, 3 Sep 2024 09:32:51 -0700 From: "Paul E. McKenney" <paulmck@...nel.org> To: rcu@...r.kernel.org Cc: 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: [PATCH rcu 0/11] Add light-weight readers for SRCU Hello! This series provides light-weight readers for SRCU. This lightness is selected by the caller by using the new srcu_read_lock_lite() and srcu_read_unlock_lite() flavors instead of the usual srcu_read_lock() and srcu_read_unlock() flavors. Although this passes significant rcutorture testing, this should still be considered to be experimental. There are a few restrictions: (1) If srcu_read_lock_lite() is called on a given srcu_struct structure, then no other flavor may be used on that srcu_struct structure, before, during, or after. (2) The _lite() readers may only be invoked from regions of code where RCU is watching (as in those regions in which rcu_is_watching() returns true). (3) There is no auto-expediting for srcu_struct structures that have been passed to _lite() readers. (4) SRCU grace periods for _lite() srcu_struct structures invoke synchronize_rcu() at least twice, thus having longer latencies than their non-_lite() counterparts. (5) Even with synchronize_srcu_expedited(), the resulting SRCU grace period will invoke synchronize_rcu() at least twice, as opposed to invoking the IPI-happy synchronize_rcu_expedited() function. (6) Just as with srcu_read_lock() and srcu_read_unlock(), the srcu_read_lock_lite() and srcu_read_unlock_lite() functions may not (repeat, *not*) be invoked from NMI handlers (that is what the _nmisafe() interface are for). Although one could imagine readers that were both _lite() and _nmisafe(), one might also imagine that the read-modify-write atomic operations that are needed by any NMI-safe SRCU read marker would make this unhelpful from a performance perspective. All that said, the patches in this series are as follows: 1. Rename srcu_might_be_idle() to srcu_should_expedite(). 2. Introduce srcu_gp_is_expedited() helper function. 3. Renaming in preparation for additional reader flavor. 4. Bit manipulation changes for additional reader flavor. 5. Standardize srcu_data pointers to "sdp" and similar. 6. Convert srcu_data ->srcu_reader_flavor to bit field. 7. Add srcu_read_lock_lite() and srcu_read_unlock_lite(). 8. rcutorture: Expand RCUTORTURE_RDR_MASK_[12] to eight bits. 9. rcutorture: Add reader_flavor parameter for SRCU readers. 10. rcutorture: Add srcu_read_lock_lite() support to rcutorture.reader_flavor. 11. refscale: Add srcu_read_lock_lite() support using "srcu-lite". Thanx, Paul ------------------------------------------------------------------------ Documentation/admin-guide/kernel-parameters.txt | 4 b/Documentation/admin-guide/kernel-parameters.txt | 8 + b/include/linux/srcu.h | 21 +- b/include/linux/srcutree.h | 2 b/kernel/rcu/rcutorture.c | 28 +-- b/kernel/rcu/refscale.c | 54 +++++-- b/kernel/rcu/srcutree.c | 16 +- include/linux/srcu.h | 86 +++++++++-- include/linux/srcutree.h | 5 kernel/rcu/rcutorture.c | 37 +++- kernel/rcu/srcutree.c | 168 +++++++++++++++------- 11 files changed, 308 insertions(+), 121 deletions(-)
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