----- On Apr 29, 2020, at 12:52 PM, rostedt rost...@goodmis.org wrote: > On Wed, 29 Apr 2020 18:20:26 +0200 > Joerg Roedel <jroe...@suse.de> wrote: > >> On Wed, Apr 29, 2020 at 06:17:47PM +0200, Joerg Roedel wrote: >> > On Wed, Apr 29, 2020 at 10:07:31AM -0400, Steven Rostedt wrote: >> > > Talking with Mathieu about this on IRC, he pointed out that my code does >> > > have a vzalloc() that is called: >> > > >> > > in trace_pid_write() >> > > >> > > pid_list->pids = vzalloc((pid_list->pid_max + 7) >> 3); >> > > >> > > This is done when -P1,2 is on the trace-cmd command line. >> > >> > And that buffer is written to at any function entry? >> >> What I meant to say, is it possible that the page-fault handler does not >> complete because at its beginning it calls into trace-code and faults >> again on the same address? >> > > It should be read only at sched_switch. > > Basically, it's a big bitmask, where each bit represents a possible process > id (can be 2 gigs if we allow all positive ints!).
I think you mean 2 giga-bit, for 256MB worth of memory, right ? And AFAIU the PID_MAX_LIMIT is at a maximum of 4 million PIDs in include/linux/threads.h, which means 512MB worth of memory for a bitmask. > Then, it is only written when setting it up. Bits 1 and 2 are set here > (-P1,2). At context switch, next->pid is checked against this bitmask, and > if it is set, it means we should allow this process to be traced. > > This mask should only be accessed at sched_switch time, not at other times. > And it may read any possible page in that mask depending on the process id > of the next task to be scheduled in. Not sure how relevant it is, but I notice that it is also touched from IPI context, see: on_each_cpu(ignore_task_cpu, tr, 1); Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
