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!). 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. -- Steve
