linux-kernel - Re: [PATCH v2] Pre-emption control for userspace

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Date:	Tue, 25 Mar 2014 11:56:31 -0600
From:	Khalid Aziz <khalid.aziz@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	peterz@...radead.org, andi.kleen@...el.com, rob@...dley.net,
	viro@...iv.linux.org.uk, oleg@...hat.com,
	gnomes@...rguk.ukuu.org.uk, riel@...hat.com, snorcht@...il.com,
	dhowells@...hat.com, luto@...capital.net, daeseok.youn@...il.com,
	ebiederm@...ssion.com, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH v2] Pre-emption control for userspace

On 03/25/2014 11:44 AM, Andrew Morton wrote:
> So the procfs file is written in binary format and is read back in
> ascii format.  Seems odd.
>
> Perhaps this should all be done as a new syscall rather than some
> procfs thing.
>

I didn't want to add yet another syscall which will then need to be 
added to glibc, but I am open to doing it through a syscall if that is 
the consensus.

>> +	struct preempt_delay {
>> +		u32 __user *delay_req;		/* delay request flag pointer */
>> +		unsigned char delay_granted:1;	/* currently in delay */
>> +		unsigned char yield_penalty:1;	/* failure to yield penalty */
>> +	} sched_preempt_delay;
>
> The problem with bitfields is that a write to one bitfield can corrupt
> a concurrent write to the other one.  So it's your responsibility to
> provide locking and/or to describe how this race is avoided.  A comment
> here in the definition would be a suitable way of addressing this.
>

I do not have a strong reason to use a bitfield, just trying to not use 
any more bytes than I need to. If using a char is safer, I would rather 
use safer code.

>> +	if (delay_req) {
>> +		int ret;
>> +
>> +		pagefault_disable();
>> +		ret = __copy_from_user_inatomic(&delay_req_flag, delay_req,
>> +				sizeof(u32));
>> +		pagefault_enable();
>
> This all looks rather hacky and unneccesary.  Can't we somehow use
> plain old get_user() and avoid such fuss?

get_user() takes longer and can sleep if page fault occurs. I need this 
code to be very fast for it to be beneficial and am willing to ignore 
page faults since page fault would imply the task has not touched 
pre-emption delay request field and hence we can resched safely.

>> +#else
>> +#define delay_resched_task(curr) resched_task(curr)
>
> This could have been implemented in C...
>

Sure, I can do that.

Thanks, Andrew!

--
Khalid

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