Re: [PATCH V2 9/9] jump_label: Batch up if arch supports it

[Steven Rostedt]

On Tue, 18 Dec 2018 17:46:38 +0100
Daniel Bristot de Oliveira <bristot@xxxxxxxxxx> wrote:


I'd say add this file first, before x86 supports it. That way it's easy
for you to test if this file is correct for other archs that do not
support it.

When x86 supports it, the "on switch" for that should be the added
config, just like what other architectures will do.

> If the architecture supports the batching of jump label updates, use it!
> 
> An easy way to see the benefits of this patch is switching the
> schedstats on and off. For instance:
> 
> -------------------------- %< ---------------------------- 
> #!/bin/bash
> 
> while [ true ]; do 
>     sysctl -w kernel.sched_schedstats=1
>     sleep 2
>     sysctl -w kernel.sched_schedstats=0
>     sleep 2
> done
> -------------------------- >% ----------------------------  
> 
> while watching the IPI count:
> 
> -------------------------- %< ---------------------------- 
> # watch -n1 "cat /proc/interrupts | grep Function"
> -------------------------- >% ----------------------------  
> 
> With the current mode, it is possible to see +- 168 IPIs each 2 seconds,
> while with this patch the number of IPIs goes to 3 each 2 seconds.
> 
> Regarding the performance impact of this patch set, I made two measurements:
> 
>     The time to update a key (the task that is causing the change)
>     The time to run the int3 handler (the side effect on a thread that
>                                       hits the code being changed)
> 
> The schedstats static key was chosen as the key to being switched on and off.
> The reason being is that it is used in more than 56 places, in a hot path. The
> change in the schedstats static key will be done with the following command:
> 
> while [ true ]; do
>     sysctl -w kernel.sched_schedstats=1
>     usleep 500000
>     sysctl -w kernel.sched_schedstats=0
>     usleep 500000
> done
> 
> In this way, they key will be updated twice per second. To force the hit of the
> int3 handler, the system will also run a kernel compilation with two jobs per
> CPU. The test machine is a two nodes/24 CPUs box with an Intel Xeon processor
> @2.27GHz.
> 
> Regarding the update part, on average, the regular kernel takes 57 ms to update
> the schedstats key, while the kernel with the batch updates takes just 1.4 ms
> on average. Although it seems to be too good to be true, it makes sense: the
> schedstats key is used in 56 places, so it was expected that it would take
> around 56 times to update the keys with the current implementation, as the
> IPIs are the most expensive part of the update.
> 
> Regarding the int3 handler, the non-batch handler takes 45 ns on average, while
> the batch version takes around 180 ns. At first glance, it seems to be a high
> value. But it is not, considering that it is doing 56 updates, rather than one!
> It is taking four times more, only. This gain is possible because the patch
> uses a binary search in the vector: log2(56)=5.8. So, it was expected to have
> an overhead within four times.
> 
> (voice of tv propaganda) But, that is not all! As the int3 handler keeps on for
> a shorter period (because the update part is on for a shorter time), the number
> of hits in the int3 handler decreased by 10%.
> 
> The question then is: Is it worth paying the price of "135 ns" more in the int3
> handler?
> 
> Considering that, in this test case, we are saving the handling of 53 IPIs,
> that takes more than these 135 ns, it seems to be a meager price to be paid.
> Moreover, the test case was forcing the hit of the int3, in practice, it
> does not take that often. While the IPI takes place on all CPUs, hitting
> the int3 handler or not!
> 
> For instance, in an isolated CPU with a process running in user-space
> (nohz_full use-case), the chances of hitting the int3 handler is barely zero,
> while there is no way to avoid the IPIs. By bounding the IPIs, we are improving
> a lot this scenario.
> 
> Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> Cc: Zhou Chengming <zhouchengming1@xxxxxxxxxx>
> Cc: Jiri Kosina <jkosina@xxxxxxx>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
> Cc: Chris von Recklinghausen <crecklin@xxxxxxxxxx>
> Cc: Jason Baron <jbaron@xxxxxxxxxx>
> Cc: Scott Wood <swood@xxxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: Clark Williams <williams@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  include/linux/jump_label.h |  3 +++
>  kernel/jump_label.c        | 29 +++++++++++++++++++++++++++++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
> index c88d903befc0..ed51ef3e1abd 100644
> --- a/include/linux/jump_label.h
> +++ b/include/linux/jump_label.h
> @@ -219,6 +219,9 @@ extern void arch_jump_label_transform(struct jump_entry *entry,
>  				      enum jump_label_type type);
>  extern void arch_jump_label_transform_static(struct jump_entry *entry,
>  					     enum jump_label_type type);
> +extern int arch_jump_label_transform_queue(struct jump_entry *entry,
> +					    enum jump_label_type type);
> +extern void arch_jump_label_transform_apply(void);
>  extern int jump_label_text_reserved(void *start, void *end);
>  extern void static_key_slow_inc(struct static_key *key);
>  extern void static_key_slow_dec(struct static_key *key);
> diff --git a/kernel/jump_label.c b/kernel/jump_label.c
> index 22093b5f57c9..08ace142af0a 100644
> --- a/kernel/jump_label.c
> +++ b/kernel/jump_label.c
> @@ -409,6 +409,7 @@ bool jump_label_can_update_check(struct jump_entry *entry, bool init)
>  	return 0;
>  }
>  
> +#ifndef HAVE_JUMP_LABEL_BATCH
>  static void __jump_label_update(struct static_key *key,
>  				struct jump_entry *entry,
>  				struct jump_entry *stop,
> @@ -421,6 +422,34 @@ static void __jump_label_update(struct static_key *key,
>  		}
>  	}
>  }
> +#else
> +static void __jump_label_update(struct static_key *key,
> +				struct jump_entry *entry,
> +				struct jump_entry *stop,
> +				bool init)
> +{
> +	for_each_label_entry(key, entry, stop) {
> +
> +		if (!jump_label_can_update_check(entry, init))
> +			continue;
> +
> +		if (arch_jump_label_transform_queue(entry,
> +						    jump_label_type(entry)))
> +			continue;
> +
> +		/*
> +		 * Queue's overflow: Apply the current queue, and then
> +		 * queue again. If it stills not possible to queue, BUG!
> +		 */
> +		arch_jump_label_transform_apply();
> +		if (!arch_jump_label_transform_queue(entry,
> +						     jump_label_type(entry))) {
> +			BUG();

Why BUG()? Do you really want to crash Linus's machine?

-- Steve

> +		}
> +	}
> +	arch_jump_label_transform_apply();
> +}
> +#endif
>  
>  void __init jump_label_init(void)
>  {