user/sven/linux.git/include/linux/srcutree.h, branch v6.16.1

srcu: Make SRCU-fast also be NMI-safe

2025-02-05T15:12:06Z

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 Signed-off-by: Paul E. McKenney Signed-off-by: Boqun Feng

srcu: Add SRCU-fast readers

2025-02-05T15:12:05Z

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 Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

srcu: Move SRCU Tree/Tiny definitions from srcu.h

2025-02-05T15:12:05Z

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 Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

srcu: Pull integer-to-pointer conversion into __srcu_ctr_to_ptr()

2025-02-05T15:12:05Z

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 Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

srcu: Pull pointer-to-integer conversion into __srcu_ptr_to_ctr()

2025-02-05T15:12:05Z

This commit abstracts the srcu_read_lock*() pointer-to-integer conversion into a new __srcu_ptr_to_ctr(). This will be used in rcutorture for testing an srcu_read_lock_fast() that returns a pointer rather than an integer. Signed-off-by: Paul E. McKenney Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

srcu: Rename srcu_check_read_flavor_lite() to srcu_check_read_flavor_force()

2025-02-05T15:12:05Z

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 Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

srcu: Make Tree SRCU updates independent of ->srcu_idx

2025-02-05T15:12:05Z

This commit makes Tree SRCU updates independent of ->srcu_idx, then drop ->srcu_idx. Signed-off-by: Paul E. McKenney Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

srcu: Make SRCU readers use ->srcu_ctrs for counter selection

2025-02-05T15:12:05Z

This commit causes SRCU readers to use ->srcu_ctrs for counter selection instead of ->srcu_idx. This takes another step towards array-indexing-free SRCU readers. [ paulmck: Apply kernel test robot feedback. ] Co-developed-by: Z qiang Signed-off-by: Z qiang Signed-off-by: Paul E. McKenney Tested-by: kernel test robot Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

srcu: Pull ->srcu_{un,}lock_count into a new srcu_ctr structure

2025-02-05T15:12:05Z

This commit prepares for array-index-free srcu_read_lock*() by moving the ->srcu_{un,}lock_count fields into a new srcu_ctr structure. This will permit ->srcu_index to be replaced by a per-CPU pointer to this structure. Signed-off-by: Paul E. McKenney Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

Merge branches 'fixes.2024.12.14a', 'rcutorture.2024.12.14a', 'srcu.2024.12.14a' and 'torture-test.2024.12.14a' into rcu-merge.2024.12.14a

2024-12-14T16:32:26Z

fixes.2024.12.14a: RCU fixes rcutorture.2024.12.14a: Torture-test updates srcu.2024.12.14a: SRCU updates torture-test.2024.12.14a: Adding an extra test, fixes