Messages in this thread | | | Date | Mon, 6 Mar 2023 13:01:06 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86/resctrl: avoid compiler optimization in __resctrl_sched_in |
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On Fri, Mar 03, 2023 at 03:11:33PM -0800, Stephane Eranian wrote:
> The problem is located in the __resctrl_sched_in() routine which rewrites > the active closid via the PQR_ASSOC register. Because this is an expensive > operation, the kernel only does it when the context switch involves tasks > with different CLOSID. And to check that, it needs to access the current > task's closid field using current->closid. current is actually a macro > that reads the per-cpu variable pcpu_hot.current_task. > > After an investigation by compiler experts, the problem has been tracked down > to the usage of the get_current() macro in the __resctrl_sched_in() code and > in particular the per-cpu macro: > > static __always_inline struct task_struct *get_current(void) > { > return this_cpu_read_stable(pcpu_hot.current_task); > } > > And as per percpu.h: > > /* > * this_cpu_read() makes gcc load the percpu variable every time it is > * accessed while this_cpu_read_stable() allows the value to be cached. > * this_cpu_read_stable() is more efficient and can be used if its value > * is guaranteed to be valid across cpus. The current users include > * get_current() and get_thread_info() both of which are actually > * per-thread variables implemented as per-cpu variables and thus > * stable for the duration of the respective task. > */ > > The _stable version of the macro allows the value to be cached, meaning it > does not force a reload.
Right, so afaict the difference between this_cpu_read() and this_cpu_read_stable() is the volatile qualifier.
this_cpu_read() is asm volatile(), while this_cpu_read_stable() and raw_cpu_read() are both an unqualified asm().
Now, afaiu we're inlining all of this into __switch_to(), which has raw_cpu_write(pcpu_hot.current_task, next_p).
And I suppose what the compiler is doing is lifting the 'current' load over that store, but how is it allowed that? I thought C was supposed to have PO consistency, That raw_cpu_write() should be seen as a store to to pcpu_hot.current_task, why can it lift a load over the store?
Specifically, percpu_to_op() has a "+m" output constaint while percpu_stable_op() has a "p" input constraint on the same address.
Compiler folks help?
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