Re: [PATCH 0/6] Simplify hash_resize.c

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

On Wed, Jan 16, 2019 at 12:32:52AM +0900, Akira Yokosawa wrote:
> On 2019/01/14 17:33:37 -0800, Paul E. McKenney wrote:
> > On Sun, Jan 13, 2019 at 06:10:24PM -0800, Paul E. McKenney wrote:
> >> On Mon, Jan 14, 2019 at 08:28:27AM +0900, Akira Yokosawa wrote:
> >>> >From 7b69a9b37ba9a73a50aad5cbb097235ddfe75870 Mon Sep 17 00:00:00 2001
> >>> From: Akira Yokosawa <akiyks@gmail.com>
> >>> Date: Mon, 14 Jan 2019 07:25:14 +0900
> >>> Subject: [PATCH 0/6] Simplify hash_resize.c
> >>>
> >>> Hi Paul,
> >>>
> >>> This patch set updates hash_resize.c, which you suggested for me to
> >>> take over, and the related text.
> >>>
> >>> I added a couple of Quick Quizzes as well, and in one of them,
> >>> I included your version of hash_resize.c.
> >>>
> >>> Patch #1 updates hash_resize.c in the way I suggested. Note that
> >>> in this update, "#ifndef FCV_SNIPPET" blocks are used to hide
> >>> code for debugging (hash value checks) in code snippets.
> >>> This code can actually be compiled with "-DFCV_SNIPPET" to see
> >>> the performance without hash value checks.
> >>>
> >>> Patch #2 adds a couple of Quick Quizzes. Your version of hash_resize.c
> >>> is added as hash_resize_s.c.
> >>>
> >>> Patch #3 removes unnecessary folding in a code snippet.
> >>>
> >>> Patch #4 adjusts a few sentence to the simpler approach.
> >>>
> >>> Patch #5 adds another Quick Quiz.
> >>>
> >>> Patch #6 adds an "#error" directive for the lack of rcu_barrier().
> >>>
> >>> All of the updates in text would need your native eyes to be polished.
> >>
> >> Nice!  Queued and pushed, thank you!  I will review and send any
> >> needed update by end of tomorrow, Pacific Time.
> > 
> > Here is a summary of changes:
> > 
> > o	Move the ht_lock_state structure definition to follow the ht
> > 	structure, matching the order of discussion in the text.
> > 	Everything seems to build and run OK with this change.
> > 	Also illustrates the advantages of line labels!  ;-)
> > 
> > o	I considered inlining ht_search_bucket() into its only caller
> > 	in hashtab_lookup(), but decided against it.  More flexibility
> > 	for change with it as is.
> > 
> > o	I added ht_search_bucket() to the in-text list of things
> > 	permitting lookups and modifications to run concurrently with
> > 	a resize operation.
> > 
> > o	I tweaked the description of hashtab_lock_mod(), hashtab_unlock_mod(),
> > 	hashtab_add(), and hashtab_del().
> > 
> > o	I tweaked the QQ asking about searches missing adds during resizes.
> > 
> > o	English is strange, so "Less Changes" must become "Fewer Changes".
> 
> Sigh, I thought I knew this rule... Thanks for the fix.
> 
> > 
> > o	I put the long labels on a single line out of paranoia.
> > 
> > I pushed these out on a working branch named paulmck.2019.01.14a.  Please
> > let me know whether I messed anything up, and once it looks good to you,
> > I will pull these two commits into the master branch.
> 
> There is an irrelevant clause added in an answer to a QQ.
> See below for my suggestion. It doesn't sound fluent enough, though.

Good catch!

How does the following amended patch look?

							Thanx, Paul

------------------------------------------------------------------------

commit 731c855ab11b9601a40324c6ef6b5ced6d7833e9
Author: Akira Yokosawa <akiyks@gmail.com>
Date:   Tue Jan 15 23:52:27 2019 +0900

    datastruct/hash: Fix irrelevant clause in answer to quick quiz
    
    hashtab_lookup() is not affected by the reordering of ->hbp[] and
    ->hls_idx[]. Instead, mention the increase of cost at the earlier
    explanation of hashtab_lookup().
    
    Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
    [ paulmck: Wordsmithing. ]
    Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

diff --git a/datastruct/datastruct.tex b/datastruct/datastruct.tex
index d1fde2c7535e..985021e927a8 100644
--- a/datastruct/datastruct.tex
+++ b/datastruct/datastruct.tex
@@ -1189,13 +1189,15 @@ a concurrent resize operation.
 	either the old bucket if it is not resized yet, or to the new
 	bucket if it has been resized, and \co{hashtab_del()} removes
 	the specified element from any buckets into which it has been inserted.
-	\co{hashtab_lookup()} searches the new bucket if the search
-	of the old bucket fails.
+	The \co{hashtab_lookup()} function searches the new bucket
+	if the search of the old bucket fails, which has the disadvantage
+	of adding overhead to the lookup fastpath.
 	The alternative \co{hashtab_lock_mod()} returns the locking
 	state of the new bucket in \co{->hbp[0]} and \co{->hls_idx[0]}
-	if resize operation is in progress.
-	This reordering simplifies \co{hashtab_add()}, but at the expense
-	of an extra check in \co{hashtab_lookup()}.
+	if resize operation is in progress, instead of the perhaps
+	more natural choice of \co{->hbp[1]} and \co{->hls_idx[1]}.
+	However, this less-natural choice has the advantage of simplifying
+	\co{hashtab_add()}.
 
 	Further analysis of the code is left as an exercise for the reader.
 } \QuickQuizEnd