On Wed, Apr 23, 2025 at 05:17:31PM +0200, Benjamin Berg wrote:
> Hi,
>
> On Wed, 2025-01-08 at 10:04 +0100, Sebastian Andrzej Siewior wrote:
> > __module_text_address() can be invoked within a RCU section, there is no
> > requirement to have preemption disabled.
> >
> > Replace the preempt_disable() section around __module_text_address()
> > with RCU.
>
> Unfortunately, this patch causes a performance regression for us. The
> trouble is that we enable kmemleak and run trace-cmd so a lot of stack
> traces need to be collected. Obviously, we also have lockdep enabled.
>
> Now, combine this with the UML stack dumping code calling into
> __kernel_text_address a lot[1] and it really has a relevant performance
> impact. I saw the kernel spending 40% of its own CPU time just on the
> lock in is_module_text_address.
>
> Maybe kernel_text_address should leave the RCU handling to the caller
> and assume that the RCU read lock is already taken?
>
> Benjamin
>
> [1] The UM arch dump_stack function reads every "unsigned long" on the
> stack and tests it using __kernel_text_address.
Use of a single guard(rcu)() is regressing performance? Interesting and
quite unexpected. That said, tiven the amount of debug you have enabled,
I am not so sure that people are going to be all that excited about a
further performance regression.
But is this regression due to the cleanup hook that guard(rcu)()
registers? If so, please feel free to try using rcu_read_lock()
and rcu_read_unlock() instead. I would be surprised if this makes a
difference, but then again, your initial regression report also comes
as a surprise, so...
Another way to reduce guard(rcu)() overhead is to build your kernel
with CONFIG_PREEMPT_NONE=y. Not so good for real-time response, but
then again, neither are your debug options.
Thanx, Paul
> > Acked-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> > Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
> > ---
> > kernel/module/main.c | 16 +++++-----------
> > 1 file changed, 5 insertions(+), 11 deletions(-)
> >
> > diff --git a/kernel/module/main.c b/kernel/module/main.c
> > index 80877741ac7e5..6a99076146cbc 100644
> > --- a/kernel/module/main.c
> > +++ b/kernel/module/main.c
> > @@ -823,13 +823,12 @@ void symbol_put_addr(void *addr)
> >
> > /*
> > * Even though we hold a reference on the module; we still
> > need to
> > - * disable preemption in order to safely traverse the data
> > structure.
> > + * RCU read section in order to safely traverse the data
> > structure.
> > */
> > - preempt_disable();
> > + guard(rcu)();
> > modaddr = __module_text_address(a);
> > BUG_ON(!modaddr);
> > module_put(modaddr);
> > - preempt_enable();
> > }
> > EXPORT_SYMBOL_GPL(symbol_put_addr);
> >
> > @@ -3694,20 +3693,15 @@ struct module *__module_address(unsigned long
> > addr)
> > */
> > bool is_module_text_address(unsigned long addr)
> > {
> > - bool ret;
> > -
> > - preempt_disable();
> > - ret = __module_text_address(addr) != NULL;
> > - preempt_enable();
> > -
> > - return ret;
> > + guard(rcu)();
> > + return __module_text_address(addr) != NULL;
> > }
> >
> > /**
> > * __module_text_address() - get the module whose code contains an
> > address.
> > * @addr: the address.
> > *
> > - * Must be called with preempt disabled or module mutex held so that
> > + * Must be called within RCU read section or module mutex held so
> > that
> > * module doesn't get freed during this.
> > */
> > struct module *__module_text_address(unsigned long addr)
>
