linux-kernel - Re: [PATCH] microblaze: Remove noMMU code

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Date:   Wed, 25 Nov 2020 14:01:15 +0100
From:   Michal Simek <michal.simek@...inx.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Michal Simek <michal.simek@...inx.com>
CC:     Mike Rapoport <rppt@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michal Simek <monstr@...str.eu>, git <git@...inx.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Baoquan He <bhe@...hat.com>,
        "Christian Brauner" <christian.brauner@...ntu.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Eric Biggers <ebiggers@...gle.com>,
        "Geert Uytterhoeven" <geert@...ux-m68k.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ira Weiny <ira.weiny@...el.com>, Jens Axboe <axboe@...nel.dk>,
        Joe Perches <joe@...ches.com>,
        Kees Cook <keescook@...omium.org>,
        Manish Narani <manish.narani@...inx.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Randy Dunlap" <rdunlap@...radead.org>,
        Rob Herring <robh@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Stafford Horne <shorne@...il.com>,
        Stefan Asserhall <stefan.asserhall@...inx.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] microblaze: Remove noMMU code



On 25. 11. 20 13:57, Arnd Bergmann wrote:
> On Wed, Nov 25, 2020 at 1:14 PM Michal Simek <michal.simek@...inx.com> wrote:
>>
>> And second part. I know I removed that Kconfig macros for it but maybe
>> good to talk about what needs to change to support different page size
>> for systems with MMU.
> 
> I would recommend just leaving it hardwired to 4K. The benefits of larger
> pages are rather questionable, especially on systems without tons of
> RAM, and it's an endless source of problems.
> 
> The commit that added the option, ba9c4f88d747 ("microblaze: Allow
> PAGE_SIZE configuration"), was rather nebulous with "can improve
> performance on some workloads".
> 
> On PowerPC, the benefits of larger pages are mainly for the hashed
> page table code, which I don't think exists anywhere else.

Is there any standard benchmark to see if there is really any benefit?
Exchanging tbl is quite expensive operation that's why there could be
good improvement.

But not a problem to remove it for now and get it back when this is
properly tested.

Thanks,
Michal