Growth inside an RRSP or RRIF is not taxed by the US until you take money out. This deferral is automatic under Rev. Proc. 2014-55; you no longer file Form 8891.
Your contributions don't reduce your US taxable income. Withdrawals are taxed by the US as pension income, minus your US basis, and the Canadian tax on them counts toward your foreign tax credit. The accounts still go on your FBAR and Form 8938.
Growth: deferred automatically
Article XVIII(7) of the US-Canada tax treaty lets a US citizen or resident defer US tax on income that builds up inside a Canadian retirement plan until it is paid out. Since Rev. Proc. 2014-55, eligible people are treated as having made that election automatically. Form 8891 is obsolete.
You are an "eligible individual" if you have filed your US returns, never reported the RRSP's inside growth as income, and reported any withdrawals as if the deferral applied. If you once reported the growth each year, you are not eligible and need IRS permission to switch.
Catching up? If you never filed, the IRS says people who come in through the Streamlined Foreign Offshore Procedures get relief consistent with Rev. Proc. 2014-55 (Streamlined FAQ 3). See Catching up on missed years.
Contributions
RRSP contributions lower your Canadian tax, not your US tax. Your US return shows your full Canadian wages. The treaty has narrow rules that can allow a US deduction for some employer pension plans, which are claimed with Form 8833; they don't apply to an ordinary personal RRSP.
Because the US never gave you a deduction, those contributions can become US basis: money that comes back to you tax-free on the US side when you withdraw. Keep your contribution records.
Withdrawals: T4RSP and T4RIF
The IRS treats RRSP and RRIF payments as pensions (Publication 597). They go on Form 1040 lines 5a and 5b: the full payment on 5a, the taxable part (payment minus your US basis) on 5b.
The Canadian tax you pay on a withdrawal counts toward your foreign tax credit. Most practitioners put RRSP and RRIF withdrawals in the general category. The IRS has not ruled on this specifically.
Reporting the accounts
Rev. Proc. 2014-55 also exempts RRSPs and RRIFs from Forms 3520 and 3520-A. It does not exempt them from the FBAR or Form 8938: include each plan's balances there.
Mutual funds and ETFs held inside an RRSP or RRIF don't need Form 8621. The PFIC rules exempt funds held through a pension arrangement covered by a tax treaty. See Mutual funds and ETFs.