From: Alan Huang <mmpgouride@gmail.com>
To: Akira Yokosawa <akiyks@gmail.com>
Cc: perfbook@vger.kernel.org, paulmck@kernel.org
Subject: Re: [PATCH] CodeSamples/count: change full memory barrier to partial one
Date: Tue, 4 Apr 2023 23:14:12 +0800 [thread overview]
Message-ID: <CB055288-6725-4518-A34A-0B75CDD1BC1E@gmail.com> (raw)
In-Reply-To: <c84751a3-3237-a3f3-3c60-5d1caaff19db@gmail.com>
Hi Akira,
Thanks for your advice!
I’ll try to fix that.
Thanks,
Alan
> 2023年4月4日 下午7:28，Akira Yokosawa <akiyks@gmail.com> 写道：
>
> Hi Alan,
>
> Thanks for your continued efforts.
> Unfortunately, I (as a LaTeX adviser) see a couple of problems.
> Please see below.
>
> On Tue, 4 Apr 2023 01:19:21 -0400, Alan Huang wrote:
>> The original memory barriers in count_stat_eventual.c ensure
>> writing to global_count happens before writing to stopflag and
>> reading from stopflag happens before later reading from global_count.
>>
>> Thus, smp_load_acquire and smp_store_release will suffice.
>>
>> In count_lim_sig.c, there is only one ordering required, that is
>> writing to counter happens before setting theft to THEFT_READY in
>> add_count/sub_count's fast path. Therefore, partial memory barrier
>> will suffice.
>>
>> Signed-off-by: Alan Huang <mmpgouride@gmail.com>
>> ---
>> CodeSamples/count/count_lim_sig.c | 10 +++-------
>> CodeSamples/count/count_stat_eventual.c | 6 ++----
>> 2 files changed, 5 insertions(+), 11 deletions(-)
>>
>> diff --git a/CodeSamples/count/count_lim_sig.c b/CodeSamples/count/count_lim_sig.c
>> index 59da8077..c2f61197 100644
>> --- a/CodeSamples/count/count_lim_sig.c
>> +++ b/CodeSamples/count/count_lim_sig.c
>> @@ -56,12 +56,10 @@ static void flush_local_count_sig(int unused) //\lnlbl{flush_sig:b}
>> {
>> if (READ_ONCE(theft) != THEFT_REQ) //\lnlbl{flush_sig:check:REQ}
>> return; //\lnlbl{flush_sig:return:n}
>> - smp_mb(); //\lnlbl{flush_sig:mb:1}
>
> Removing a line that has //\lnlbl{foo} from a code snippet will cause
> warnings of "Reference ... undefined on ...".
>
> I think you will see warnings that look like:
>
> ----- Excerpt around remaining warning messages -----
> [63]
> <count/sig-theft.pdf, id=5442, 172.645pt x 234.8775pt>
> File: count/sig-theft.pdf Graphic file (type pdf)
> <use count/sig-theft.pdf>
> Package pdftex.def Info: count/sig-theft.pdf used on input line 2303.
> (pdftex.def) Requested size: 172.64456pt x 234.87692pt.
> [64 <./count/sig-theft.pdf>] (./CodeSamples/count/count_lim_sig@data.fcv)
>
> LaTeX Warning: Reference `ln:count:count_lim_sig:migration:flush_sig:mb:1' on p
> age 65 undefined on input line 2428.
>
> [...]
>
> You need to update text-side references to the to-be-removed label.
> You will likely need to adjust wording in surrounding text as well.
>
> If you are wondering how these labels in code snippets can be
> referenced from LaTeX sources, please see Appendix D.3.1.1.
>
>> WRITE_ONCE(theft, THEFT_ACK); //\lnlbl{flush_sig:set:ACK}
>> if (!counting) { //\lnlbl{flush_sig:check:fast}
>> - WRITE_ONCE(theft, THEFT_READY); //\lnlbl{flush_sig:set:READY}
>> + smp_store_release(&theft, THEFT_READY); //\lnlbl{flush_sig:set:READY}
>> }
>> - smp_mb();
>> } //\lnlbl{flush_sig:e}
>>
>> static void flush_local_count(void) //\lnlbl{flush:b}
>> @@ -125,8 +123,7 @@ int add_count(unsigned long delta) //\lnlbl{b}
>> WRITE_ONCE(counting, 0); //\lnlbl{clearcnt}
>> barrier(); //\lnlbl{barrier:3}
>> if (READ_ONCE(theft) == THEFT_ACK) { //\lnlbl{check:ACK}
>> - smp_mb(); //\lnlbl{mb}
>
> Ditto.
>
> Can you submit a v2 including text-side updates?
>
> In the meantime, I'll review your code changes.
>
> Thanks, Akira
>
>
>> - WRITE_ONCE(theft, THEFT_READY); //\lnlbl{READY}
>> + smp_store_release(&theft, THEFT_READY); //\lnlbl{READY}
>> }
>> if (fastpath)
>> return 1; //\lnlbl{return:fs}
>> @@ -164,8 +161,7 @@ int sub_count(unsigned long delta)
>> WRITE_ONCE(counting, 0);
>> barrier();
>> if (READ_ONCE(theft) == THEFT_ACK) {
>> - smp_mb();
>> - WRITE_ONCE(theft, THEFT_READY);
>> + smp_store_release(&theft, THEFT_READY);
>> }
>> if (fastpath)
>> return 1;
>> diff --git a/CodeSamples/count/count_stat_eventual.c b/CodeSamples/count/count_stat_eventual.c
>> index 967644de..7157ee0e 100644
>> --- a/CodeSamples/count/count_stat_eventual.c
>> +++ b/CodeSamples/count/count_stat_eventual.c
>> @@ -51,8 +51,7 @@ void *eventual(void *arg) //\lnlbl{eventual:b}
>> WRITE_ONCE(global_count, sum);
>> poll(NULL, 0, 1);
>> if (READ_ONCE(stopflag)) {
>> - smp_mb();
>> - WRITE_ONCE(stopflag, stopflag + 1);
>> + smp_store_release(&stopflag, stopflag + 1);
>> }
>> }
>> return NULL;
>> @@ -73,9 +72,8 @@ void count_init(void) //\lnlbl{init:b}
>> void count_cleanup(void) //\lnlbl{cleanup:b}
>> {
>> WRITE_ONCE(stopflag, 1);
>> - while (READ_ONCE(stopflag) < 3)
>> + while (smp_load_acquire(&stopflag) < 3)
>> poll(NULL, 0, 1);
>> - smp_mb();
>> } //\lnlbl{cleanup:e}
>> //\end{snippet}
next prev parent reply other threads:[~2023-04-04 15:14 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-04 5:19 [PATCH] CodeSamples/count: change full memory barrier to partial one Alan Huang
2023-04-04 11:28 ` Akira Yokosawa
2023-04-04 15:07 ` Alan Huang
2023-04-04 15:14 ` Alan Huang [this message]
-- strict thread matches above, loose matches on Subject: below --
2023-04-05 16:56 Alan Huang
2023-04-06 0:09 ` Akira Yokosawa
2023-04-06 18:26 ` Paul E. McKenney
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