Date: Fri, 22 Jul 2022 17:27:45 -0600 From: Kristof Provost <kp@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: Re: git: 151abc80cde7 - main - if_vlan: avoid hash table thrashing when adding and removing entries Message-ID: <41E75387-FE50-474E-BDC4-909A114FFD78@FreeBSD.org> In-Reply-To: <202207222319.26MNJEVY032683@gitrepo.freebsd.org> References: <202207222319.26MNJEVY032683@gitrepo.freebsd.org>
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On 22 Jul 2022, at 17:19, Kristof Provost wrote: > The branch main has been updated by kp: > > URL: https://cgit.FreeBSD.org/src/commit/?id=3D151abc80cde778bc18b91c33= 4d07fbd52bbb38fb > > commit 151abc80cde778bc18b91c334d07fbd52bbb38fb > Author: Kristof Provost <kp@FreeBSD.org> > AuthorDate: 2022-07-22 17:17:04 +0000 > Commit: Kristof Provost <kp@FreeBSD.org> > CommitDate: 2022-07-22 17:18:41 +0000 > > if_vlan: avoid hash table thrashing when adding and removing entrie= s > > vlan_remhash() uses incorrect value for b. > > When using the default value for VLAN_DEF_HWIDTH (4), the VLAN hash= -list table > expands from 16 chains to 32 chains as the 129th entry is added. tr= unk->hwidth > becomes 5. Say a few more entries are added and there are now 135 e= ntries. > trunk-hwidth will still be 5. If an entry is removed, vlan_remhash(= ) will > calculate a value of 32 for b. refcnt will be decremented to 134. T= he if > comparison at line 473 will return true and vlan_growhash() will be= called. The > VLAN hash-list table will be compressed from 32 chains wide to 16 c= hains wide. > hwidth will become 4. This is an error, and it can be seen when a n= ew VLAN is > added. The table will again be expanded. If an entry is then remove= d, again > the table is contracted. > > If the number of VLANS stays in the range of 128-512, each time an = insert > follows a remove, the table will expand. Each time a remove follows= an > insert, the table will be contracted. > > The fix is simple. The line 473 should test that the number of entr= ies has > decreased such that the table should be contracted using what would= be the new > value of hwidth. line 467 should be: > > b =3D 1 << (trunk->hwidth - 1); > > PR: 265382 > Reviewed by: kp > MFC after: 2 weeks > Sponsored by: NetApp, Inc. Submitted by: David Sips <dsips@netapp.com> I thought I=E2=80=99d set the author, but clearly not. Kristof
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