linux-kernel - Re: [PATCH v2 08/26] selftests/resctrl: Split measure_cache

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Message-ID: <f6f42f73-ef84-535-78d7-c93685625aca@linux.intel.com>
Date:   Thu, 7 Dec 2023 20:33:41 +0200 (EET)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
cc:     linux-kselftest@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
        Shaopeng Tan <tan.shaopeng@...fujitsu.com>,
        Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 08/26] selftests/resctrl: Split measure_cache_vals()

On Thu, 7 Dec 2023, Reinette Chatre wrote:
> On 12/7/2023 6:32 AM, Ilpo Järvinen wrote:
> > On Tue, 28 Nov 2023, Reinette Chatre wrote:
> >> On 11/20/2023 3:13 AM, Ilpo Järvinen wrote:
> 
> ...
> >>> -	/*
> >>> -	 * Measure llc occupancy from resctrl.
> >>> -	 */
> >>> -	if (!strncmp(param->resctrl_val, CMT_STR, sizeof(CMT_STR))) {
> >>> -		ret = get_llc_occu_resctrl(&llc_occu_resc);
> >>> -		if (ret < 0)
> >>> -			return ret;
> >>> -		llc_value = llc_occu_resc;
> >>> -	}
> >>> -	ret = print_results_cache(param->filename, bm_pid, llc_value);
> >>> -	if (ret)
> >>> +	ret = print_results_cache(filename, bm_pid, llc_perf_miss);
> >>> +	return ret;
> >>> +}
> >>
> >> Perhaps print_results_cache() can be made to return negative error
> >> and this just be "return print_results_cache(...)" and the function
> >> comment be accurate?
> > 
> > I think, I'll just change all "return errno;" to "return -1" before this,
> > however, one open question which impacts whether this is actually Fixes 
> > class issue:
> > 
> > It seems that perror()'s manpage doesn't answer one important question, 
> > whether it ifself can alter errno or not. The resctrl selftest code 
> > assumes it doesn't but some evidence I came across says otherwise so doing 
> > return errno; after calling perror() might not even be valid at all.
> > 
> > So I'm tempted to create an additional Fixes patch about the return change 
> > into the front of the series.
> > 
> 
> I would not trust errno to contain code of earlier calls after a call to perror().
> If errno is needed I think it should be saved before calling perror(). Looking
> at perror() at [1] I do not see an effort to restore errno before it returns,
> and looking at the implementation of perror() there appears to be many
> opportunities for errno to change.
> 
> Reinette
> 
> [1] https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-common/perror.c;h=51e621e332a5e2aa76ecefb3bcf325efb43b345f;hb=HEAD#l47

I already spent some moments in converting all return error -> return -1, 
since all such places do perror() calls anyway (which I also converted to 
ksft_perror() or ksft_print_msg() where perror() didn't make any sense) 
there's not much added value in returning the errno which was not 
correctly done in the existing code anyway.


-- 
 i.