forked from Minki/linux
mainlining shenanigans
8663effb24
I finally got around to creating trampolines for dynamically allocated ftrace_ops with using synchronize_rcu_tasks(). For users of the ftrace function hook callbacks, like perf, that allocate the ftrace_ops descriptor via kmalloc() and friends, ftrace was not able to optimize the functions being traced to use a trampoline because they would also need to be allocated dynamically. The problem is that they cannot be freed when CONFIG_PREEMPT is set, as there's no way to tell if a task was preempted on the trampoline. That was before Paul McKenney implemented synchronize_rcu_tasks() that would make sure all tasks (except idle) have scheduled out or have entered user space. While testing this, I triggered this bug: BUG: unable to handle kernel paging request at ffffffffa0230077 ... RIP: 0010:0xffffffffa0230077 ... Call Trace: schedule+0x5/0xe0 schedule_preempt_disabled+0x18/0x30 do_idle+0x172/0x220 What happened was that the idle task was preempted on the trampoline. As synchronize_rcu_tasks() ignores the idle thread, there's nothing that lets ftrace know that the idle task was preempted on a trampoline. The idle task shouldn't need to ever enable preemption. The idle task is simply a loop that calls schedule or places the cpu into idle mode. In fact, having preemption enabled is inefficient, because it can happen when idle is just about to call schedule anyway, which would cause schedule to be called twice. Once for when the interrupt came in and was returning back to normal context, and then again in the normal path that the idle loop is running in, which would be pointless, as it had already scheduled. The only reason schedule_preempt_disable() enables preemption is to be able to call sched_submit_work(), which requires preemption enabled. As this is a nop when the task is in the RUNNING state, and idle is always in the running state, there's no reason that idle needs to enable preemption. But that means it cannot use schedule_preempt_disable() as other callers of that function require calling sched_submit_work(). Adding a new function local to kernel/sched/ that allows idle to call the scheduler without enabling preemption, fixes the synchronize_rcu_tasks() issue, as well as removes the pointless spurious schedule calls caused by interrupts happening in the brief window where preemption is enabled just before it calls schedule. Reviewed: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20170414084809.3dacde2a@gandalf.local.home Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.