Messages in this thread | | | Date | Fri, 8 Aug 2014 08:40:20 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 tip/core/rcu 3/9] rcu: Add synchronous grace-period waiting for RCU-tasks |
| |
On Thu, Aug 07, 2014 at 05:18:23PM -0400, Steven Rostedt wrote: > On Thu, 7 Aug 2014 22:08:13 +0200 > Peter Zijlstra <peterz@infradead.org> wrote: > > > OK, you've got to start over and start at the beginning, because I'm > > really not understanding this.. > > > > What is a 'trampoline' and what are you going to use them for. > > Great question! :-) > > The trampoline is some code that is used to jump to and then jump > someplace else. Currently, we use this for kprobes and ftrace. For > ftrace we have the ftrace_caller trampoline, which is static. When > booting, most functions in the kernel call the mcount code which > simply returns without doing anything. This too is a "trampoline". At > boot, we convert these calls to nops (as you already know). When we > enable callbacks from functions, we convert those calls to call > "ftrace_caller" which is a small assembly trampoline that will call > some function that registered with ftrace. > > Now why do we need the call_rcu_task() routine? > > Right now, if you register multiple callbacks to ftrace, even if they > are not tracing the same routine, ftrace has to change ftrace_caller to > call another trampoline (in C), that does a loop of all ops registered > with ftrace, and compares the function to the ops hash tables to see if > the ops function should be called for that function. > > What we want to do is to create a dynamic trampoline that is a copy of > the ftrace_caller code, but instead of calling this list trampoline, it > calls the ops function directly. This way, each ops registered with > ftrace can have its own custom trampoline that when called will only > call the ops function and not have to iterate over a list. This only > happens if the function being traced only has this one ops registered. > For functions with multiple ops attached to it, we need to call the > list anyway. But for the majority of the cases, this is not the case. > > The one caveat for this is, how do we free this custom trampoline when > the ops is done with it? Especially for users of ftrace that > dynamically create their own ops (like perf, and ftrace instances). > > We need to find a way to free it, but unfortunately, there's no way to > know when it is safe to free it. There's no way to disable preemption > or have some other notifier to let us know if a task has jumped to this > trampoline and has been preempted (sleeping). The only safe way to know > that no task is on the trampoline is to remove the calls to it, > synchronize the CPUS (so the trampolines are not even in the caches), > and then wait for all tasks to go through some quiescent state. This > state happens to be either not running, in userspace, or when it > voluntarily calls schedule. Because nothing that uses this trampoline > should do that, and if the task voluntarily calls schedule, we know > it's not on the trampoline. > > Make sense?
Ok, so they're purely used in the function prologue/epilogue callchain. And you don't want to use synchronize_tasks() because registering a trace functions is atomic ?
But why would you use dynamic memory allocation for these trampolines at all? Why not use the one default trampoline for this?
Suppose that thing looks like:
ftrace_mcount_handler() { for_each_hlist_rcu(entry,..) entry->func(); }
so why not make it look like:
ftrace_mcount_handler() { asm_volatile_goto("jmp %l[label]" ::: &do_list); return;
do_list: for_each_hlist_rcu(entry,...) entry->func(); }
Then, for: no entries -> NOP, one entry -> "CALL $func", more entries -> "JMP &do_list.
No need for extra allocations and fancy means of getting rid of them, and only a few bytes extra wrt the existing function. [unhandled content-type:application/pgp-signature] | |