linux-kernel - Re: [PATCH 1/3] selftests/resctrl: Fix schemata write error check

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Message-ID: <viypgry5j2xk6txo6kwsqo4kmlag267krhg67oyhiooi4cd7db@ii4cin3btjo5>
Date:   Fri, 25 Aug 2023 10:50:40 +0200
From:   Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Reinette Chatre <reinette.chatre@...el.com>,
        <fenghua.yu@...el.com>
Subject: Re: [PATCH 1/3] selftests/resctrl: Fix schemata write error check

On 2023-08-25 at 11:43:22 +0300, Ilpo Järvinen wrote:
>On Fri, 25 Aug 2023, Maciej Wieczór-Retman wrote:
>> On 2023-08-24 at 15:52:05 +0300, Ilpo Järvinen wrote:
>> >Ki,
>> >
>> >You're lacking a few people from the To/Cc list. Please see KERNEL 
>> >SELFTEST FRAMEWORK entry in MAINTAINERS.
>> 
>> Thank you, I thought I checked the MAINTAINERS file well enough. I'll
>> add them in the next version
>> 
>> >On Thu, 24 Aug 2023, Wieczor-Retman, Maciej wrote:
>> >
>> >> Writing bitmasks to the schemata can fail when the bitmask doesn't
>> >> adhere to some constraints defined by what a particular CPU supports.
>> >> Some example of constraints are max length or being having contiguous
>> >
>> >"being having" is not good English.
>> 
>> Thanks, I'll change it
>> 
>> >> bits. The driver should properly return errors when any rule concerning
>> >> bitmask format is broken.
>> >> 
>> >> Resctrl FS returns error codes from fprintf() only when fclose() is
>> >> called.
>> >
>> >I wonder if this is actually related to libc doing buffering between 
>> >fprintf() and the actual write() syscall.
>> 
>> I started looking and apparently in the manpages for fclose [1] it says
>> it uses fflush() to flush any buffered data in the stream. So that would
>> probably confirm that it does buffering there.
>> 
>> In this case is there a situation when the fprintf() before fclose()
>> would report an error? I'm thinking if there is a point to keep error
>> checking after both function calls or just fclose(). 
>>
>> Or would putting additional fflush() after fprintf() make some sense?
>> To have separate error checks for both function calls.
>
>Another approach would be to use syscalls directly (open, write, and 
>close to eliminate the buffering entirely. Given schema is already 
>written into local variable first, it would be quite straightforward to do 
>that conversion.

Okay, I'll try that then, thanks

-- 
Kind regards
Maciej Wieczór-Retman