Re: [PATCH] count: Employ new scheme for snippet of count_stat_eventual.c

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Sat, Oct 06, 2018 at 07:35:42AM +0900, Akira Yokosawa wrote:
> >From 16fb7e39700268cd498f88c16b34170e3b91fd24 Mon Sep 17 00:00:00 2001
> From: Akira Yokosawa <akiyks@gmail.com>
> Date: Thu, 6 Oct 2018 07:20:06 +0900
> Subject: [PATCH] count: Employ new scheme for snippet of count_stat_eventual.c
> 
> Also omit READ_ONCE()s which access variables owned and modified
> only by the owning thread.
> 
> Read part of stopflag's increment in eventual() doesn't need
> READ_ONCE() because once the "if" statement stands, stopflag
> won't be modified by any other thread.
> 
> Also modify code around pthread_create() in count_init() so that
> the "if" statement won't be too wide for a code snippet in two-
> column layout.
> 
> Signed-off-by: Akira Yokosawa <akiyks@gmail.com>

Queued and pushed, thank you!

> ---
> Paul,
> 
> I might be missing something in omitting READ_ONCE() of stopflag's
> increment in eventual().
> If this were kernel code, we wouldn't be able to make sure
> that eventual() is the only updater after the particular point in
> execution.
> 
> Thoughts?

True enough!  If a given entity is the only thing that updates a
given variable, then that entity (but only that entity) need not use
READ_ONCE().

Hmmm... This applies to Figure 5.1 also, right?

							Thanx, Paul

>         Thanks, Akira
> --
> 
>  CodeSamples/count/count_stat_eventual.c | 38 +++++++++-------
>  count/count.tex                         | 81 +++++++--------------------------
>  2 files changed, 37 insertions(+), 82 deletions(-)
> 
> diff --git a/CodeSamples/count/count_stat_eventual.c b/CodeSamples/count/count_stat_eventual.c
> index 324bc24..fa8972f 100644
> --- a/CodeSamples/count/count_stat_eventual.c
> +++ b/CodeSamples/count/count_stat_eventual.c
> @@ -21,22 +21,24 @@
>  
>  #include "../api.h"
>  
> -DEFINE_PER_THREAD(unsigned long, counter);
> -unsigned long global_count;
> -int stopflag;
> +//\begin{snippet}[labelbase=ln:count:count_stat_eventual:whole,commandchars=\$\[\]]
> +DEFINE_PER_THREAD(unsigned long, counter);		//\lnlbl{per_thr_cnt}
> +unsigned long global_count;				//\lnlbl{glb_cnt}
> +int stopflag;						//\lnlbl{stopflag}
>  
> -void inc_count(void)
> +static __inline__ void inc_count(void)			//\lnlbl{inc:b}
>  {
> -	WRITE_ONCE(__get_thread_var(counter),
> -		   READ_ONCE(__get_thread_var(counter)) + 1);
> -}
> +	unsigned long *p_counter = &__get_thread_var(counter);
>  
> -__inline__ unsigned long read_count(void)
> +	WRITE_ONCE(*p_counter, *p_counter + 1);
> +}							//\lnlbl{inc:e}
> +
> +static __inline__ unsigned long read_count(void)	//\lnlbl{read:b}
>  {
>  	return READ_ONCE(global_count);
> -}
> +}							//\lnlbl{read:e}
>  
> -void *eventual(void *arg)
> +void *eventual(void *arg)				//\lnlbl{eventual:b}
>  {
>  	int t;
>  	unsigned long sum;
> @@ -49,29 +51,31 @@ void *eventual(void *arg)
>  		poll(NULL, 0, 1);
>  		if (READ_ONCE(stopflag)) {
>  			smp_mb();
> -			WRITE_ONCE(stopflag, READ_ONCE(stopflag) + 1);
> +			WRITE_ONCE(stopflag, stopflag + 1);
>  		}
>  	}
>  	return NULL;
> -}
> +}							//\lnlbl{eventual:e}
>  
> -void count_init(void)
> +void count_init(void)					//\lnlbl{init:b}
>  {
>  	int en;
>  	thread_id_t tid;
>  
> -	if ((en = pthread_create(&tid, NULL, eventual, NULL)) != 0) {
> +	en = pthread_create(&tid, NULL, eventual, NULL);
> +	if (en != 0) {
>  		fprintf(stderr, "pthread_create: %s\n", strerror(en));
>  		exit(EXIT_FAILURE);
>  	}
> -}
> +}							//\lnlbl{init:e}
>  
> -void count_cleanup(void)
> +void count_cleanup(void)				//\lnlbl{cleanup:b}
>  {
>  	WRITE_ONCE(stopflag, 1);
>  	while (READ_ONCE(stopflag) < 3)
>  		poll(NULL, 0, 1);
>  	smp_mb();
> -}
> +}							//\lnlbl{cleanup:e}
> +//\end{snippet}
>  
>  #include "counttorture.h"
> diff --git a/count/count.tex b/count/count.tex
> index c36e7d8..aea7b93 100644
> --- a/count/count.tex
> +++ b/count/count.tex
> @@ -785,94 +785,45 @@ converge on the true value---hence this approach qualifies as
>  eventually consistent.
>  
>  \begin{listing}[tbp]
> -{ \scriptsize
> -\begin{verbbox}
> - 1 DEFINE_PER_THREAD(unsigned long, counter);
> - 2 unsigned long global_count;
> - 3 int stopflag;
> - 4
> - 5 void inc_count(void)
> - 6 {
> - 7   WRITE_ONCE(__get_thread_var(counter),
> - 8              READ_ONCE(__get_thread_var(counter)) + 1);
> - 9 }
> -10
> -11 unsigned long read_count(void)
> -12 {
> -13   return READ_ONCE(global_count);
> -14 }
> -15
> -16 void *eventual(void *arg)
> -17 {
> -18   int t;
> -19   unsigned long sum;
> -20
> -21   while (READ_ONCE(stopflag) < 3) {
> -22     sum = 0;
> -23     for_each_thread(t)
> -24       sum += READ_ONCE(per_thread(counter, t));
> -25     WRITE_ONCE(global_count, sum);
> -26     poll(NULL, 0, 1);
> -27     if (READ_ONCE(stopflag)) {
> -28       smp_mb();
> -29       WRITE_ONCE(stopflag, READ_ONCE(stopflag) + 1);
> -30     }
> -31   }
> -32   return NULL;
> -33 }
> -34
> -35 void count_init(void)
> -36 {
> -37   int en;
> -38   thread_id_t tid;
> -39
> -40   if ((en = pthread_create(&tid, NULL, eventual, NULL)) != 0) {
> -41     fprintf(stderr, "pthread_create: %s\n", strerror(en));
> -42     exit(-1);
> -43   }
> -44 }
> -45
> -46 void count_cleanup(void)
> -47 {
> -48   WRITE_ONCE(stopflag, 1);
> -49   while (READ_ONCE(stopflag) < 3)
> -50     poll(NULL, 0, 1);
> -51   smp_mb();
> -52 }
> -\end{verbbox}
> -}
> -\centering
> -\theverbbox
> +\input{CodeSamples/count/count_stat_eventual@whole.fcv}
>  \caption{Array-Based Per-Thread Eventually Consistent Counters}
>  \label{lst:count:Array-Based Per-Thread Eventually Consistent Counters}
>  \end{listing}
>  
> +\begin{lineref}[ln:count:count_stat_eventual:whole]
>  The implementation is shown in
>  Listing~\ref{lst:count:Array-Based Per-Thread Eventually Consistent Counters}
>  (\path{count_stat_eventual.c}).
> -Lines~1-2 show the per-thread variable and the global variable that
> -track the counter's value, and line three shows \co{stopflag}
> +Lines~\lnref{per_thr_cnt}-\lnref{glb_cnt}
> +show the per-thread variable and the global variable that
> +track the counter's value, and line~\lnref{stopflag} shows \co{stopflag}
>  which is used to coordinate termination (for the case where we want
>  to terminate the program with an accurate counter value).
> -The \co{inc_count()} function shown on lines~5-9 is similar to its
> +The \co{inc_count()} function shown on
> +lines~\lnref{inc:b}-\lnref{inc:e} is similar to its
>  counterpart in
>  Listing~\ref{lst:count:Array-Based Per-Thread Statistical Counters}.
> -The \co{read_count()} function shown on lines~11-14 simply returns the
> +The \co{read_count()} function shown on
> +lines~\lnref{read:b}-\lnref{read:e} simply returns the
>  value of the \co{global_count} variable.
>  
> -However, the \co{count_init()} function on lines~35-44
> -creates the \co{eventual()} thread shown on lines~16-33, which
> +However, the \co{count_init()} function on
> +lines~\lnref{init:b}-\lnref{init:e}
> +creates the \co{eventual()} thread shown on
> +lines~\lnref{eventual:b}-\lnref{eventual:e}, which
>  cycles through all the threads,
>  summing the per-thread local \co{counter} and storing the
>  sum to the \co{global_count} variable.
>  The \co{eventual()} thread waits an arbitrarily chosen one millisecond
>  between passes.
> -The \co{count_cleanup()} function on lines~46-52 coordinates termination.
> +The \co{count_cleanup()} function on
> +lines~\lnref{cleanup:b}-\lnref{cleanup:e} coordinates termination.
>  
>  This approach gives extremely fast counter read-out while still
>  supporting linear counter-update performance.
>  However, this excellent read-side performance and update-side scalability
>  comes at the cost of the additional thread running \co{eventual()}.
> +\end{lineref}
>  
>  \QuickQuiz{}
>  	Why doesn't \co{inc_count()} in
> -- 
> 2.7.4
>