On Tue, Nov 21, 2023 at 05:50:29PM +0100, Peter Zijlstra wrote: > On Tue, Nov 21, 2023 at 11:00:13AM -0500, Mathieu Desnoyers wrote: > > On 2023-11-21 10:52, Peter Zijlstra wrote: > > > On Tue, Nov 21, 2023 at 03:46:43PM +0100, Peter Zijlstra wrote: > > > > > > > Why is this such a hard question? > > > > > > Anyway, recapping from IRC: > > > > > > preemptible, SRCU: > > > counter-array based, GP advances by increasing array index > > > and waiting for previous index to drop to 0. > > > > > > notably, a GP can pass while a task is preempted but not within a > > > critical section. > > > > > > SRCU has smp_mb() in the critical sections to improve GP. > > > > Also: > > > > preemptible only allows blocking when priority inheritance is > > guarantees, which excludes doing I/O, and thus page faults. > > Otherwise a long I/O could cause the system to OOM. > > > > SRCU allows all kind of blocking, as long as the entire SRCU > > domain does not mind waiting for a while before readers complete. > > Well, no. Fundamentally both SRCU and preemptible (and many other > flavours) are just a counter-array. The non-blocking for preempt comes > from the fact that it is the main global rcu instance and allowing all > that would make GPs too rare and cause you memory trouble. > > But that's not because of how it's implemented, but because of it being > the main global instance. > > > > tasks: > > > waits for every task to pass schedule() > > > > > > ensures that any pieces of text rendered unreachable before, is > > > actually unused after. > > > > > > tasks-rude: > > > like tasks, but different? build to handle tracing while rcu-idle, > > > even though that was already deemed bad? > > > > > > tasks-tracing-rcu: > > > extention of tasks to have critical-sections ? Should this simply be > > > tasks? > > > > tasks-trace-rcu is meant to allow tasks to block/take a page fault within > > the read-side. It is specialized for tracing and has a single domain. It > > does not need the smp_mb on the read-side, which makes it lower-overhead > > than SRCU. > > That's what it's meant for, not what it is. > > Turns out that tasks-tracing is a per-task counter based thing, and as > such does not require all tasks to pass through schedule() and does not > imply the tasks flavour (nor the tasks-rude) despite the similarity in > naming. > > But now I am again left wondering what the fundamental difference is > between a per-task counter and a per-cpu counter. > > At the end of the day, you still have to wait for the thing to hit 0. > > So I'm once again confused, ... Updating myself.. so task-tracing-rcu is in fact *very* similar to regular preemptible-rcu but is slightly different mostly because it is *not* the main global instance. Both are a single per-task counter (and not the per-cpu summing that I remember from many many *many* years ago; OLS'07), mostly because this helps identify which task is to blame when things go sideways.
