linux-kernel - Re: [vfs] 8bb3c61baf: vm-scalability.median -23.7% regression

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Date:   Sun, 8 Sep 2019 22:46:01 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     kernel test robot <rong.a.chen@...el.com>
Cc:     David Howells <dhowells@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, lkp@...org
Subject: Re: [vfs]  8bb3c61baf:  vm-scalability.median -23.7% regression

On Tue, Sep 03, 2019 at 04:41:22PM +0800, kernel test robot wrote:
> Greeting,
> 
> FYI, we noticed a -23.7% regression of vm-scalability.median due to commit:
> 
> 
> commit: 8bb3c61bafa8c1cd222ada602bb94ff23119e738 ("vfs: Convert ramfs, shmem, tmpfs, devtmpfs, rootfs to use the new mount API")
> https://kernel.googlesource.com/pub/scm/linux/kernel/git/viro/vfs.git work.mount
> 
> in testcase: vm-scalability
> on test machine: 88 threads Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz with 128G memory
> with following parameters:
> 
> 	runtime: 300s
> 	size: 16G
> 	test: shm-pread-rand
> 	cpufreq_governor: performance
> 	ucode: 0xb000036

That thing loses size=... option.  Both size= and nr_blocks= affect the
same thing (->max_blocks), but the parser keeps track of the options
it has seen and applying the parsed data to superblock checks only
whether nr_blocks= had been there.  IOW, size= gets parsed, but the
result goes nowhere.

I'm not sure whether it's better to fix the patch up or redo it from
scratch - it needs to be carved up anyway and it's highly non-transparent,
so I'm probably going to replace the damn thing entirely with something
that would be easier to follow.