Re: [RFC PATCH 1/2] pipe: introduce busy wait for pipe

[Subhra Mazumdar]

On 08/31/2018 09:09 AM, Steven Sistare wrote:
> On 8/30/2018 4:24 PM, subhra mazumdar wrote:
> > Introduce pipe_ll_usec field for pipes that indicates the amount of micro
> > seconds a thread should spin if pipe is empty or full before sleeping. This
> > is similar to network sockets. Workloads like hackbench in pipe mode
> > benefits significantly from this by avoiding the sleep and wakeup overhead.
> > Other similar usecases can benefit. pipe_wait_flag is used to signal any
> > thread busy waiting. pipe_busy_loop_timeout checks if spin time is over.
> >
> > Signed-off-by: subhra mazumdar <subhra.mazumdar@xxxxxxxxxx>
> > ---
> >   include/linux/pipe_fs_i.h | 19 +++++++++++++++++++
> >   1 file changed, 19 insertions(+)
> >
> > diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
> > index e7497c9..fdfd2a2 100644
> > --- a/include/linux/pipe_fs_i.h
> > +++ b/include/linux/pipe_fs_i.h
> > @@ -1,6 +1,8 @@
> >   #ifndef _LINUX_PIPE_FS_I_H
> >   #define _LINUX_PIPE_FS_I_H
> >   
> > +#include <linux/sched/clock.h>
> > +
> >   #define PIPE_DEF_BUFFERS	16
> >   
> >   #define PIPE_BUF_FLAG_LRU	0x01	/* page is on the LRU */
> > @@ -54,6 +56,8 @@ struct pipe_inode_info {
> >   	unsigned int waiting_writers;
> >   	unsigned int r_counter;
> >   	unsigned int w_counter;
> > +	unsigned int pipe_ll_usec;
> > +	unsigned long pipe_wait_flag;
> >   	struct page *tmp_page;
> >   	struct fasync_struct *fasync_readers;
> >   	struct fasync_struct *fasync_writers;
> > @@ -157,6 +161,21 @@ static inline int pipe_buf_steal(struct pipe_inode_info *pipe,
> >   	return buf->ops->steal(pipe, buf);
> >   }
> >   
> > +static inline unsigned long pipe_busy_loop_current_time(void)
> > +{
> > +	return (unsigned long)(local_clock() >> 10);
> Why ">> 10" ? local_lock() has nanosec units, and you compare to the tunable
> pipe_llc_sec which has microsec units.  Should be ">> 3".  Better yet, redefine
> the tunable to have nanosec units.  I suspect you will need very large values
> of the tunable to show similar results.
It's 2^10. I don't think using nanosec units is necessary. It is unlikely
data will be read or written in nano seconds. sk_busy_loop_timeout for
sockets uses micro seconds too.
> Also, since this type of optimization consumes CPU extra cycles that could
> be used by other tasks, show the overall CPU utilization before and after
> the optimization, such as by using "time hackbench ...".
OK.

Thanks,
Subhra
> - Steve
>
> > +}
> > +
> > +static inline bool pipe_busy_loop_timeout(struct pipe_inode_info *pipe,
> > +					  unsigned long start_time)
> > +{
> > +	unsigned long bp_usec = READ_ONCE(pipe->pipe_ll_usec);
> > +	unsigned long end_time = start_time + bp_usec;
> > +	unsigned long now = pipe_busy_loop_current_time();
> > +
> > +	return time_after(now, end_time);
> > +}
> > +
> >   /* Differs from PIPE_BUF in that PIPE_SIZE is the length of the actual
> >      memory allocation, whereas PIPE_BUF makes atomicity guarantees.  */
> >   #define PIPE_SIZE		PAGE_SIZE