On Tue, Sep 23, 2025 at 7:21 AM Paul E. McKenney <[email protected]> wrote: > > This commit updates the documentation to declare that RCU Tasks Trace > is implemented as a thin wrapper around SRCU-fast. > > Signed-off-by: Paul E. McKenney <[email protected]> > Cc: Andrii Nakryiko <[email protected]> > Cc: Alexei Starovoitov <[email protected]> > Cc: Peter Zijlstra <[email protected]> > Cc: <[email protected]> > --- > .../RCU/Design/Requirements/Requirements.rst | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > > diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst > b/Documentation/RCU/Design/Requirements/Requirements.rst > index f24b3c0b9b0dc6..4a116d7a564edc 100644 > --- a/Documentation/RCU/Design/Requirements/Requirements.rst > +++ b/Documentation/RCU/Design/Requirements/Requirements.rst > @@ -2779,12 +2779,12 @@ Tasks Trace RCU > ~~~~~~~~~~~~~~~ > > Some forms of tracing need to sleep in readers, but cannot tolerate > -SRCU's read-side overhead, which includes a full memory barrier in both > -srcu_read_lock() and srcu_read_unlock(). This need is handled by a > -Tasks Trace RCU that uses scheduler locking and IPIs to synchronize with > -readers. Real-time systems that cannot tolerate IPIs may build their > -kernels with ``CONFIG_TASKS_TRACE_RCU_READ_MB=y``, which avoids the IPIs at > -the expense of adding full memory barriers to the read-side primitives. > +SRCU's read-side overhead, which includes a full memory barrier in > +both srcu_read_lock() and srcu_read_unlock(). This need is handled by > +a Tasks Trace RCU API implemented as thin wrappers around SRCU-fast, > +which avoids the read-side memory barriers, at least for architectures > +that apply noinstr to kernel entry/exit code (or that build with > +``CONFIG_TASKS_TRACE_RCU_NO_MB=y``.
For my own education (and due to laziness to try to figure this out on my own), what's the situation where you'd want to stick to the old-school "heavy-weight" SRCU vs SRCU-fast variant? > > The tasks-trace-RCU API is also reasonably compact, > consisting of rcu_read_lock_trace(), rcu_read_unlock_trace(), > -- > 2.40.1 >
